Reason always loses in amiga land. MorphOS developers tend to belittle AROS, claim that it is messy, that it has no apps, that it is mostly broken etc. Yet, where did they go when they needed to get rid of the need for OS3.1 binaries?
Well, there are still not many apps compared to 3.x, MorphOS or OS4, even if the situation has quite improved in the last months (which might even allow someone to use it for "real", with a few key apps like a browser, a video player, ...).
Obviously they can't be blamed for that. The developers have clearly focused on writing drivers and making the OS run on as much hardware as possible, which is a tedious task, leaving little time to write actual applications, or even polish existing codebase (hence numerous bugs and basic functionality in most components, like console, preferences, desktop, ...).
The situation is the opposite on MorphOS (or even OS4 to some extent), where the supported hardware is quite limited, which gives more time to polish the system, where components have more functionality and where most bugs could be ironed out. And obviously, the fact they had transparent 68k emulation since the start made the initial lack of applications much more bearable (i still don't see AROS janus uae as a real solution for 68k integration, except for games).
This is where the latter approach has an advantage... It gives an usable system (you could always argue on it, but i don't care

for daily use, even if it's on outdated hardware. Having a half-finished system on modern hardware can't attract many end users. There was probably noone using exclusively AROS when there was neither a browser nor a video player, for instance.
And of course, you're right that mac hardware for MorphOS or x1000/SAM for OS4 will only help for a couple years. Then drastic changes will have to be made anyway, if they are to survive on modern hardware.
Regarding the comparison with UAE, the answer is easy: OS3.x is dead, and very few (real) applications are still written for it. The development is clearly targetted for the new OSes.