Why do you not have backward compatibility when using UAE? You only have a problem with f.e. libraries that only exist for 68k and cannot be ported. I do not know how often this is the case and how many software needs this. And of course you have a problem with components written in assembler and thus not be portable. But if MorphOS dev are looking in future they have avoided this.
I for example have still lots of 68k libraries, mui classes, shell commands, devices, datatypes, filesystems, commodities, system enhancement programs etc in use on my MorphOS setups. Many of them aren't available for porting anymore...
And then I want to use same ARexx ports, clipboards, filesystem, network etc with 68k apps. I don't know if there's something done for those in AROS "sandboxes" for example. Of course UAE is fine for games etc, but these kinds of resource sharing things come in my mind for productivity use. Not to talk about GUI part if application has several windows/screens etc? I wouldn't like to use them less integrated to main system...