If everyone used OS4 then the majority of OS4 users would be boring, looking at the same standard background and windows, etc. The reason Amiga installations are unique is because Amiga users are unique.
OS4 is never going to appeal to the masses. OS4 is not going to replace PCs and Macs. OS4 would, if it ran on say PPC Mac Minis, be a great hobby operating system for Amiga enthusiasts, and that's enough.
The lack of memory protection is a serious flaw, there's no two ways about it. All I want is to be able to play with OS4 at night when I'm bored.