With mmu, every app has its own 32bit address space, and the mmu “maps” the app space to physical space. Legacy apps get shared address space, while newer apps get isolated space. To port apps, its only the messaging system that needs rework, and if the system does its magic with same API, transition is invisible. At the end of the day, only the message address space needs to be shared. Sendmessage() behind the scenes remaps mmu addresses. Not impossible, just requires the will to do it.

The problem with that is that the messaging system permeates everything on the Amiga, so you'd lose basically every app without rework. That might have worked if the Amiga had a growing market share, but at the time this became an option, the Amiga market was already dying and most users still only had 68000 based machines without MMU until long after Commodore died.

It's not "only" the message address space, as e.g. messages included pointers to a lot of other data structures that'd now either need to be copied or mapped into the receiving process, or the message formats reworked.