The Amiga chipset used DMA very extensively, so the CPU's memory access speed is not as important as on other machines. There were other architectural constraints, like the fact that the CPU had to compete with such chips for access to the same memory. So-called Fast memory was not a default setup for years. And the true solution, dual-ported RAM would have been too expensive.
Which games do you think were constrained by the CPU? I can think of other factors first:
- sloppy ports
- it was a more complex system to tame than most of those that preceded it
- the sprites were simply too limited, so you had to use the Blitter a lot
- unlike a lot of chips of the era, Denise had no tile or character generator so, again, you had to use the Blitter
- the CPU was clocked at 7.14MHz to align with the PAL/NTSC clock even if it could run at 8MHz
- if you used more than four bitplanes in low resolution, the Agnus chip would let Denise starve the CPU even on cycles where the latter would normally have access (if you set the right bit, the Blitter could be allowed to win, too)
I don't see how a different CPU would have helped with any of the above. In a time period during which memory wasn't typically the limit, the Amiga was one of the first systems where the CPU starved for it.
The X68000 was better, sure, but it came out later and, being in the same mold as the arcade systems of the era, it had dedicated video RAM, tiles and more powerful, actually useful sprites.
