The developers support isn't just a minor quibble, especially when it comes to quality of life factors like browser versions (e.g. LG, HTC, Samsung, etc. have all shipped Chrome builds with subtle differences). This also applies to optimizations for CPUs, graphics hardware, etc. — the app may be available on both but it almost certainly runs better on iOS.

The other problem is that fragmentation increases the odds of a vendor walking away from a device since the per-model sales are divided more ways. Trying to pick a device which will get updates basically involves sales forecasting which wouldn't be necessary if the platform was open.

Fragmentation impacts which device you might want to buy. The biggest argument for Android is the wide choice of devices and features on those devices, but if your phone never gets an update, you're left behind. It's possible to get a really nice and inexpensive Android phone, but what's the risk of never getting an update?

Not only that, but you can quickly solve that issue by going with Pixel/Nexus devices. So I'm not sure that the criticism is well-deserved.

It certainly is an issue if the entire Android ecosystem of devices is reduced to just one niche $650 phone.

Sure, but you are also comparing it to a 700$ phone (the iPhone) and I don't think it's fair.

Like, obviously, iOS will have a cleaner and less fragmented ecosystem, because they support far fewer devices (all of which they own).