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.