I believe MOS has some pretty good support for Amiga applications, AOS4... not so great, AROS for the moment seems all over the place, though it should be noted that AROS code can be found in AOS from 3.5 onwards.
The problem with dropping support for legacy code is that it can be argued that Amiga's sole asset at this point is it's software library.
Without it, it's just another ultra niche OS with poor to no hardware support and naff all else.
The problem is that the way AmigaOS was made, is that it simply cannot be brought up to date without (for all intents and purposes) a completely clean slate at this point. I've read several times on here recently from people with far greater understanding of this then I, that the AmigaOS as we knew it was pretty much end of life by the time that Commodore (or possibly Escom, though that's pushing it) closed it's doors.
Love it for what it is: A great little OS that does wonders with very few resources.
The idea of AmigaOS as being in a position to take over the world or even become a major player at this point died a long, long time ago.