I strongly believe that if they had replaced just one library at a time on a 68k system using all other stock libraries, we'd have a stable, fast 68k Aros by now.
Not really.. It is impossible to replace "hardware/low level level" libraries (like exec, expansion, graphics, intuition, layer etc..) just one by one because AmigaOS does not really have any kind of real private data, many libraries access other libraries undocumented/private data directly or do other undocumented things.
Result would have been AmigaOS clone that has exact same limitations and restrictions (see below).
Impossible. AROS is totally different internally and has built-in low level graphics driver support. AROS can have different low level device driver for different graphics mode IDs automatically, without any kind of stupid patching. This is not possible with AmigaOS. (and which makes AmigaOS RTG support total mess!)