Yes, it would be much better if everyone used one aspect ratio, one dpi, one device - but this is something you just deal with, if you're building a native app, not complain about it, or even expect that screen-sizes will forever be constant (which is what the OP suggested). It's like if web developers complained that their job is more difficult because they have to support users resizing their browser window. Strictly true, but insane.
Again, iOS developers got spoiled by not having to deal with this stuff for the last few years so maybe it seems like it's a big deal. The reality is, once you stop expecting a specific screen properties, it really isn't that big of a deal. It's just reality.
And no, it's not fragmentation. There is absolutely no reason to freeze out certain users because of their screen-size, or not provide certain features.
Calm down silly-generalization-man. This my surprise you but "iOS developers" aren't a monolith, aren't spoiled, and most have developed for other environments as well. There are more sizes to develop and test for. What was one is now more than one. Fragmentation.
Again, iOS developers got spoiled by not having to deal with this stuff for the last few years so maybe it seems like it's a big deal. The reality is, once you stop expecting a specific screen properties, it really isn't that big of a deal. It's just reality.
And no, it's not fragmentation. There is absolutely no reason to freeze out certain users because of their screen-size, or not provide certain features.