I'd assume it would have been pretty similar but we'd be running on Motorola CPU's (or a descendant of them)

The platforms would have needed to be opened up to clone builders to reach critial mass.

Amiga was a lot like DOS/Classic Mac OS, single user, unprotected memory...we would have seen it added on to like Windows 3.x/9x, until a re-written version with the right stuff took over (like Windows NT/2000/XP did).

Someone like Linus would have still likely written a UNIX clone and open-sourced it.

The minicomputers and UNIX servers/workstations would have still hung around for a while. The real trick is the Amiga CPU and the rest of the hardware would need to keep getting improved, catch up in speed, reach 64 bits, SMP...