This. This is the answer.

Shadow, IcaroRibeiro, Jumpin, VideoGameAccountant, all good answers. 

and the couch coop is not prevalent likewise handheld Nintendo is.  The Arcade games and pc mobile games have different outputs, directions, and different focus on features of the games. The Nintendo handheld has the Arcade DNA and mobile have more PC DNA than Arcade( online focus and not couch coop) 

Soon we have 8 billion people on the planet with different tastes. I think if it's good "everything" can be a success to some degree. There might be just not enough space for different competitors.

Did he really say the Wii U had no Monster Hunter when all it takes is a simple google search?

The Wii U had no "new" Monster Hunter to drive sales in Japan. 3 Ultimate came out on 3DS first, so most people didn't see the need to buy a whole new console to play the same game even though it had cross save/play. Now if the Wii U got it's own exclusive Monster Hunter then sales would have exploded.

The Switch wouldn't be nearly as successful if it wasn't a handheld as well as a home console. Would it be a flop if it was either a handheld or a home console only? Perhaps, but that'd be speculation.

I'm so happy to see so many people finally waking up to the idea that mobile didn't kill handhelds. I'm quoting OP here, but not for the purposes of a rebuttal. I think his post is right. Just commenting on what he said. 

First, it was repeatedly stated that the rise of mobile phones had eaten so deeply into the handheld market that dedicated handhelds post Nintendo DS and PSP were doomed.

Yeah, people definitely thought this, but the 3DS still sold 76 million units, which is way more sales than "Nintendoomed!" So those people were wrong even back in 2015. 

Yet even still, due to that number being less than half of DS lifetime sales, it seemed to support that mobile had taken a massive chunk of the dedicated handheld market away for good.

A lot of people just don't take into account that the DS was a tough act to beat. Look at PS2 sales ending at 158 million. Now look at PS4 sales (which will probably only ever hit 120 million). PS4 is likely to only hit 76% of the PS2's lifetime sales total. If 3DS had sold 76% of DS lifetime sales it would only be at 117 million units. So 37 million units of the sales gap between DS and 3DS can already be accounted for by the fact that DS was the greatest handheld ever made (saleswise).

Many on this very forum attribute the success of the Nintendo Switch purely to its capability to be used as a
handheld. But how can that be true, if mobile phones already killed the dedicated handheld market, a market which
has been shrinking for years?

The answer is that mobile didn't kill the handheld market. Sales in Japan are definitely due to Switch's prowess as a handheld device. Tradition consoles have been on a decline there for decades. 

The main problem that the 3DS and Vita had was that they were both too expensive for the handheld market. Let's look at handheld launch prices adjusted for inflation.

Gameboy launched at $89.99, which is $190.88 in today's money. 

Atari Lynx launched at $179.99, which is $381.77 in today's money.

Sega GameGear launched at $149.99, which is $301.83 in today's money. 

TurboExpress launched at $249.99. which is $503.27 in today's money.

Gameboy Pocket launched at $69.99, which is $117.33 in today's money.

Gameboy Color launched at $69.99, which is $112.93 in today's money. 

GBA launched at $99.99, which is $148 in today's money. 

GBA SP launched at $99.99, which is $142.92 in today's money.

DS launched at $149.99, which is $208.84 in today's money. 

PSP launched at $249.99, which is $336.67 in today's money. 

DS Lite launched at $129.99, which is $169.59 in today's money. 

Vita Launched at $249.99, (NA price/date) which is $286.38 in today's money. 

3DS launched at $249.99, (N/A price/date) which is $292.31 in today's money. 

3DS XL launched at $199.99, which is $229.10 in today's money. 

2DS launched at $129.99, which is $146.76 in today's money. 

2DS XL launched at $149.99, which is $160.94 in today's money. 

Clearly the handheld market favored the GBC, GBA, and DS Lite which all had a price point under $170 in today's money. Once you pass that $170 pricepoint you start to hit a sales ceiling of around 80 million units (even with an amazing library and fantastic support). This is why 3DS and PSP both failed to break past 82 million units, despite having excellent libraries. They simply cost too much money. 

Now, some people might point out the prices of the 2DS and 2DS XL are both under $170 in today's money. Why didn't those models sell well? 2DS lacked the clamshell design and was inferior to the 3DS in every way. Other Nintendo handheld redesigns improved on the previous model vastly. As for the 2DS XL, it launched in July of 2017, and was too late to make a difference. The Switch was already on it's way to replacing the 3DS/2DS as Nintendo's newest handheld.

If Nintendo had launched the 2DS XL model in 2011 as the ONLY model of the 3DS/2DS for $139.99, then we would be looking at 117 million sales instead of 76 million sales for that system. Edit: I forgot to scale this price down to account for inflation. I changed the price from $169.99 to $139.99 to account for inflation. 

3DS had a poor launch in terms of software. The 3D effect was rumored to hurt the eyes of people with glasses and small children. Nintendo launched the 3DS XL with a price cut, betraying early adopters of the 3DS. Vita had a poor software lineup its entire life. This combination of price and launch slipups is what killed the Vita and 3DS' sales numbers, not mobile gaming. 

But even still, with the failure of the Wii U, it had been stated that the "core" Nintendo base had been arrived at. Around 14 million who would buy any console Nintendo puts out, and around 76 million handheld users who would do the same. This reasoning led to a belief that even as a hybrid system, the Switch couldn't possibly sell more than 90 million units. Which is something that it is already poised to do later this year.

That reasoning didn't even take into account that many Wii U owners were 3DS owners and vice versa. But yeah, I agree that people capping Switch at 90 million were not using their heads.

Why would the version that is more expensive due to its "least desirable feature" sell out first? 

Joycon drift. That's why. :P

And a great game it is! I wished they had ported this to Switch initially instead of Generations.

I think what people generally misunderstand is the concept of "markets". They assume there is a console-market and a handheld market and therefore mobile games can steal the players from the handheld market. But the reality is, that there are a lot of people with differing interests. For some of these people these interests align, so they often purchase similar devices. But that is much more segmented, than just two or three markets.

Mobile gaming offered a lot no handheld or console ever could. So they got a lot of new customers that hadn't bought a console before. But for some people the handhelds before were a good enough compromise for their interests and now mobile offered them a better option. These people were lost to handhelds. But I doubt they make a majority of the former purchasers of handhelds.

A successful device often plays to multiple possible interests and therefore can interest more people. So I think 3DS offered less of the experiences the DS offered and had a higher price, so not all DS-owners converted. It is similar to how the WiiU couldn't capture the audience of the Wii. Switch offers a lot more. Alone the feature to be played on TV or as a handheld obvioulsy caters to different interests and can so get different people. The games and their diversity rope in more. Look at the diversity of gaming offering in Switch's first year from Nintendo: Zelda, 1-2-Switch, Mario Kart, Splatoon 2, Super Mario Odyssey, Arms and Xenoblade 2. That covers a lot of different interests and explains why Switch could start so successful.

Switch killed the idea that handheld consoles and stationary consoles (and the often forgotten tabletop consoles) are separate entities, even though it was Nintendo itself that gave birth to this impression when the NES and GB became so popular.
People who have their head stuck in past conventions cannot fully understand the Switch success.