It would be nice to get REAL 2D Metroid for once. Actual sprites. Hell, let fuckin' Vanillaware or someone make it. ANYONE who could and would give us a truly Super Metroid type experience again. Enough of this "2.5D" shit.

On a powerful console 2.5D is far superior. On a weak console like the 3DS I agree however, sprites would have been a far more sensible choice. 

I felt like Samus Returns was basically a test run where Nintendo wanted to see if MercurySteam under Sakamoto's supervision would create a good game, and if so they'd pursue another Metroid game under similar circumstances. So if nothing else I hope this rumour is true so that I can be proven right.

So Yokai Watch Buster 2 sold not even one-third of the first, Ever Oasis bombed, The Alliance Alive sold only over a half of The Legend of Legacy despite better game in every way and Lady Layton also sold a lot worse than other main games. All are new games that still sold bad even Pokemon and Fire Emblem, why Metroid is different? You should know Metroid Prime 1, 2 & 3 each sold near 1,5 mil already. Fewer people buy Trilogy Prime when they already have the 1, 2 on GC (Wii is backward compatible to GC) and 3 was released before the trilogy came out. Also, many gamers hate motion control so why do they have to purchase the same game with the control they dislike?

Yokai Watch Buster 2 was like the 10th Yokai Watch game in the 3DS in 4 years, you could easily put that one under 'franchise fatigue' and Lady Layton had a cheaper version for IOS/Android that launched at the same time as the 3DS one. The Alliance Alive deserved better, yeah, but its numbers aren't bad, it still probably is one of Furyu's best selling games to date. The only one that sadly bombed hard is Ever Oasis, a new IP, but there are other new IP's that bombed even harder before the Switch was even a thing, remember Code Name S.T.E.A.M. and Metroid Prime: Federation Forces?

On the other hand, lazy efforts like Mario Party: The Top 100, Pokemon Ultra Sun/Ultra Moon, Kirby: Battle Royale and Mario & Luigi: Superstar Saga still made bank. Fire Emblem Warriors bombed everywhere, but the 3DS version still managed to sell about half of the heavily pushed, far better looking version available for the shiny new console.

About Metroid Prime Trilogy, lets not forget that the Gamecube - Wii situation was similar to the current Wii U - Switch one, if games like Pokken Tournament and Mario Kart 8 will likely outsell the originals just by adding the DLC in the cartridge and little more, Metroid Prime Trilogy that had 3 higly praised games in a single package should definitely have done better.

Except many hardcore gamers hate Wii while they love Switch. Second, remember Switch is a portable at core, not home console like Wii. The Alliance Alive is nowhere The Legend of Legacy which sold 53,974, roughly half of it despite much better story, characters, gameplay and budget also. Yokai Watch Buster 2 was like the 10th Yokai Watch game, that mean all games have the 10th game is franchise fatigue, right?  Lady Layton had a cheaper version for IOS/Android that launched at the same time as the 3DS one, so are all Ace Attorney games on 3ds and except Apollo Justice, it didn't sold that low, Lady Layton is a new game, not a remake also. Metroid Prime: Federation Forces will bomb on anything, you know? Code Name S.T.E.A.M. have good gameplay, but people look over it because very bad art style and mediocre presentation. Fire Emblem Echoes's sale underperform too (131,668), lower than Shadow Dragon on DS (144,905). Fire Emblem Warriors sold low on Switch, 3ds ver sold half of it (even if sold more than bad a bit) is nothing to praise about

Avoid that 'hardcore gamers' bullshit, anyone that feels entitled to disregard and look down on things because in his little mind likes to think he is superior than others is an idiot. "Oh, I'm a hardcore gamer, I love Metroid Prime, but Wii... nah, I'm not touching that, motion controls, casuals, not gonna play it",  laughable. Also, Switch is more Wii's (amazing console) succesor than Wii U's, with the added benefit of portability.

To address the rest of your points:

* The Alliance Alive is a game I'm eagerly awaiting and as I said before it certainly deserved better sales in Japan, but it's also a FuRyu game and they always sell like crap, so the numbers are not really that bad. As you also said, it came after The Legend of Legacy, wich wasn't exactly stellar and may have given a reason to potential buyers to shy away from it.

* On Yokai Watch, the number of games released in 4 years is the problem, the time lapse between one release and the other is what kept diminishing its sales potential, I can't remember any other franchise being milked that hard in recent times.

* Ace Attorney 5 and 6 for IOS/Android were late ports, the games were exclusive to the 3DS for quite some time, that didn't happen with Lady Layton. Lady Layton launched at the same time for IOS/Andoird/3DS in Japan and since the mobile version launched WW on that day, the release for 3DS in the west actually came later and at a higher price point.

* So Metroid Prime: Federation Forces will bomb on anything and Code Name S.T.E.A.M had mediocre presentation, you're telling me that they were new IP's that nobody asked for and for that reason were doomed from the start, why can't Ever Oasis be clasified as such? Just to be clear here, I own both Code Name S.T.E.A.M and Ever Oasis and enjoy them, I'm not trying to mock the games. On the other hand, a new IP like Miitopia seems to have clicked well, that one is probably at 800k by now, I dare say, not bad at all.

* I know that VGChartz numbers for software aren't exactly accurate, but they have Echoes at 590k. Lets just say that in reality it sold 400k, do you think in undeperformed for what the game is, a remake of what's considered by many as the worst FE, using gameplay mechanics that were already dated since the launch of Awakening in 2012?

* The point I was trying to make with the Fire Emblem Warriors comparison is that the 3DS version that was barely even shown and was clearly not the one they were trying to sell, managed to represent about a third of the total sales so far, I wonder wich version's performance Koei Tecmo should be sadder with.

"On Yokai Watch, the number of games released in 4 years is the problem, the time lapse between one release and the other is what kept diminishing its sales potential, I can't remember any other franchise being milked that hard in recent times". There is: Pokemon from 2012 to 2016 https://www.gamefaqs.com/search_advanced

"I know that VGChartz numbers for software aren't exactly accurate, but they have Echoes at 590k. Lets just say that in reality it sold 400k, do you think in underperformed for what the game is, a remake of what's considered by many as the worst FE, using gameplay mechanics that were already dated since the launch of Awakening in 2012?" Shadow Dragon was a lot worse, story, characters and gameplay. Gaiden can be considered one of the worst but not the worst and sales was a bit lower than Shadow Dragon

"So Metroid Prime: Federation Forces will bomb on anything and Code Name S.T.E.A.M had mediocre presentation, you're telling me that they were new IP's that nobody asked for and for that reason were doomed from the start, why can't Ever Oasis be classified as such? Just to be clear here, I own both Code Name S.T.E.A.M and Ever Oasis and enjoy them, I'm not trying to mock the games. On the other hand, a new IP like Miitopia seems to have clicked well, that one is probably at 800k by now, I dare say, not bad at all" Ever Oasis can sold a lot better if it have a Switch ver also.

Almost all 3ds games last year are already too late for big sales because Switch. You say Echoes in reality sold 400k have nothing to be considered underperformed then Metroid Samus Return is the same. It came out after Federation Force and very late on 3ds when Switch was released already but it still manage to get ~ 500.000 units so what Snoorlax said Metroid should sold a lot better on a user base 70 million of 3ds and won't sell 1 million on Switch are nonsense and premature

Pokemon from 2012 - 2016 has less releases than Yo-Kai Watch, in a bigger period of time, for two different handhelds instead of one. Pokemon is also a long running series, while Yo-Kai Watch has only been around for 4 years. If Level 5 doesn't take a break with the series they'll run it to the ground, and I guess that's where The Snack World comes in.

You disregarded my comment about Miitopia being a new IP that found success while launching late in the 3DS life. That might point that Ever Oasis simply wasn't as attractive, in the same vein as Federation Forces an Code Name S.T.E.A.M weren't. What makes you believe that the game would have done better on the Switch? Why those Switch owners interested in the game skipped it for the platform where it's actually available?? 

Shadow Dragon was the only Fire Emblem released for the DS in the west, it was in a better situation than Echoes from the start. As for the actual numbers, I was just lowballing VGChartz estimate for Echoes, the only number we know for certain are Japan's sales, and I don't think they're bad. About Samus Returns and future sales of the Prime series, that's a discusion you have with another user, I never said that Samus Returns sales were bad nor did I imply that Prime 4 will have a lackluster performance, my only comment on that matter was about Metroid Prime Trilogy underperforming on the Wii, and that's an undeniable fact.

Almost all 3ds games released by Nintendo last year and at the start of this one were cheap cash grabs that didn't even deserve the sales they got. Do you believe that absolute trash like Mario Party: The Top 100 and Kirby Battle Royale should be selling like Breath of The Wild? Switch will affect 3DS sales, that much is obvious, but should Nintendo decide to actually give a damn about what they're releasing in the platform the games can still pull decent numbers, Echoes, Samus Returns and Miitopia have show that.

Moar Metroid can only be a good thing.