The thing is you can't really pick and choose things like this ... you could say the GBA's success was even more impressive because it came before Japan really started to replace their console usage with handhelds. 

GBA sales weren't "bizarrely" high really anyway. They were in line with the GBC yearly sales. DS had a bit of a slow launch, but it then took off too. 

GBC, GBA, and DS all pulled their weight of yearly shipments in the 14-15 million range (DS obviously went well beyond that at its peak). The 3DS is where we start seeing first real declines in Nintendo's yearly handhelds/sold average for the first time since the mid-1990s when the original Game Boy was fizzling out (pre-Pokemon) and Nintendo slapped together the Virtual Boy to try and placate that. 

The other disturbing thing about the 3DS is that it didn't really gain any further sales traction even with better releases over time and more model revisions, like a normal sales cycle (ie: release of Pokemon, new revisions, etc.). It basically peaked its first full year on market and has been declining every year since, which indicates something (read: smartphones) have attached themselves like an anchor to the 3DS' foot and is slowly but surely causing it to sink. 

All that said, Nintendo can't afford to have handheld shipments sink to 1993-1996 levels ... they had the Super NES back then ... they don't have that today. So that train of logic crashes into that brick wall pretty quick. 

I think it has a place, as its home to full, fleshed out games and aren't littered with f2p models and pay-walls.

We'll really just have to wait and see.

You're seeing it right now. This year will be Nintendo's lowest shipment in handhelds in almost 20 years (lowest since before the creation of Pokemon) even with a recent model revision. 

Sony's userbase went from 80 million users to 10 million maybe. 

This isn't a "maybe" any more. People who don't think smartphones are having any big impact have their head so deep in the sand it's not even funny, I mean even Nintendo, the most stubborn company out there basically caved and swallowed their pride to make smartphone games. It's obvious what's happening. 

You could see this coming for a long time anyway, you just had to get off the internet and open your eyes ... any time you go to any airport you'll see the amount of kids playing on tablet/phone outnumbers the kids playing on a 3DS/DS/Vita/PSP by like 10:1, if not more than that. 

Yes but ur acting like a single generation of decline means that it will inevitably continue to decline which may not be the case.

There won't be another generation of portables, 100% sure about Sony, maybe Nintendo will try, but will be a huge failure, the new 3DS, an early new generation console was a smart move to make more money.

Other than 3D screens which added to the cost of the system, what are the stupid design choices Nintendo made with 3DS?

The indicators right now are not good though. The 3DS keeps declining year over year no matter what Nintendo throws at it ... Pokemon, Animal Crossing, model revisions, 2D Mario, 3D Mario, Mario Kart, etc. etc. etc. 

I think this basically is the new baseline for Nintendo handhelds ... 6-11 mill handhelds/year. 

U can't honestly believe there won't be another generation of portables, 3DS is on track to sell something like 65-70 million with multiple 10 million selling 1st party titles, Nintendo isn't just going to abandon those sales.

It basically is a DS2. Even if you don't like the 3D, no game requires it and it takes 1 second to turn it off. The $250 thing doesn't really carry much water with me because it was only that price for like 3 months of its entire lifecycle. 

Nintendo made a lot of mistakes with the GBA and DS too ... the GBA screen was *horrible* lol. I remember it was pretty much unusable unless you have direct sunlight or were directly underneath a light to play it. People were going to the trouble of risking destroying their GBA to try and mod it by adding lights to it before Nintendo released the GBA SP. 

The DS' first year was really slow too and the design was ugly and bulky.