What you're saying is technically true, but for the vast majority of people it's about the convenience of not having to carry a second device.
In the same way that a real camera is better than a smartphone, convenience trumps quality.
>> If you're looking for something cheap to carry your retro games around on, might I recommend a GBA?
I have a 3DS because I wanted to play Street Fighter, Tekken and Dead Or Alive on the go. But I don't always bring it with me. In fact, I only bring it with me on vacations. And when I'm out on the go with just my phone, well, I don't play anything at all - it's not a huge loss. FWIW, you can already do everything you mention and more on a 2DS/3DS with the right "accessories" and a little bit of time.
Price aside, it's actually easy. Just buy a micro-SD card and the right "accessory" (there are maybe only two worth buying) and it takes a few minutes. With the most popular accessory, you can also run any home brew .3ds files very easily.
It only gets hard if you want to play online, which means you need to do .CIA files, which becomes difficult (it also requires a new SD card). But if playing online isn't a priority, it's not even an issue.
In the same way that a real camera is better than a smartphone, convenience trumps quality.
>> If you're looking for something cheap to carry your retro games around on, might I recommend a GBA?
I have a 3DS because I wanted to play Street Fighter, Tekken and Dead Or Alive on the go. But I don't always bring it with me. In fact, I only bring it with me on vacations. And when I'm out on the go with just my phone, well, I don't play anything at all - it's not a huge loss. FWIW, you can already do everything you mention and more on a 2DS/3DS with the right "accessories" and a little bit of time.