Lots of people have said that here over the years (and a quick google lead me to plenty of examples on other forums). If there's one thing you can be certain of with each new generation, it's that every possible in and cross gen comparison will be made, and that every possible conclusion (no matter how silly) will be arrived at :p

is this sarcasm? tons of people said that for the entire 7th gen !!

something tells me your emotional investment has reached unhealthy levels

While many people might rag on the 3DS I feel like it's done it's best in the climate that it finds itself. Yeah I know that HH sales are a shadow of their former selves but we really can't fully blame Nintendo on that can we? As a parallel example I would point to the drop off between the PS2 and PS3 as another "failed to live up to expectations" story. Hopefully going forward Nintendo will find some way to adapt to the current situation and find a stronger foothold.

The 3ds sold great in an era that people buy mobile games

It's not a failure by any means, it's just disappointing in terms of hardware. Software on the other hand is a different story.

DS aside, because the DS was the second greatest selling gaming system of all-time, second only to the PlayStation 2. And a majority of that audience, as well as the PSP's audience, went to the Smartphone, mobile gaming market.

I mean, the 3DS has just about everything you could ask for from a Nintendo Handheld.
-Pokemon, Mario, in all forms and genres, Zelda, Animal Crossing, Donkey Kong (while a port of a Wii game, is still more than what the DS got), Fire Emblem, Star Fox, Kirby, the successful return of Kid Icarus, it even got Smash Bros.
-Respectable 3rd Party Support from major Japanese publishers such as Capcom (Monster Hunter, Phoenix Wright, Resident Evil), Square Enix (Kingdom Hearts, and various other RPGs), SEGA (Sonic), Bandai Namco (Tales of).
-And despite all of that, it is tracking well below where the DS was when that system was at the point of it's life where the 3DS is now.

But, I wouldn't say the 3DS is failure because of that, not at all. If anything, that goes to show how much of a Grand Slam, walk-off, success that the DS was. And the casual market was a big part of that, but now mobile and smartphone devices have taken that market away almost entirely.

So for a better comparison, let's try comparing the 3DS to the GameBoy Advance.
I don't think it's going to match the 81 million units of the GameBoy Advance.
I'd say the 3DS will finish somewhere around 70 million units. Just below the mark.
However, whatever the 3DS loses to the GameBoy Advance in hardware, I think it will make up for in software.
I think when all is said and done, the 3DS will finish with better software sales figures overall, than the GameBoy Advance. And with that, the 3DS will be a big success in it's own right, and will stand proudly shoulder to shoulder with the GameBoy line as they all look above their head at the DS.

3DS had already 50+ mio sales, more than any other of current dedicated gaming systems (consoles and handhelds) will ever reach.

Vita is a completely other subject. Vita hard and software sales are extremely low and Sony abandoned the handheld early. For example the Wii U has low hardware sales too, but at least already many successful games and million sellers.

Also, the 3DS has a better software distribution than the DS. The DS has hundreds of wonderful games, but even with a +150million userbase, a lot of those hardly broke the million mark. We even have cases like SMT Devil Survivor, whose remake sold better on the 3DS than on the DS, despite having  1/3 of the userbase. Games like Chinatown, Pokemon Conquest and Fire Emblem: Shadow Dragon would have sold better on the 3DS if they were avalible, they are pretty hard to find nowadays. The lack of a DS Virtual Console on the 3DS still baffles me.

Especially considering the explosion in popularity for some of those franchises that you mentioned. SMT and Fire Emblem.
Virtual Console re-releases of most of those games would've helped them out big time, for sure.

It's partly fair to say the DS was an anomaly because like the Wii, it appealed to a lot of casuals who were unlikely to become longtime gamers.
I think the 3DS sales are good enough. The only bad thing about the 3DS is that they seem to have given up making big first party games rather soon. If they'd had another year or 2 with big releases, they could probably got it up to about 70 million, which would be a success. Whereas it'll probably only hit a max of 60 million now.

People also like to ignore the GBA. I think it could have been a 100 million seller easily if Nintendo didn't cut it short and went with DS.