The view at Symbolics was mostly (AFAIK) that 32bit were not enough for their software - their microprocessor was a 40bit + 8bit ECC architecture. They also ported their software to the DEC Alpha, because that was a 64bit machine.

But generally I would agree that the 68020 + MMU would be mostly okay - given that MCL then ran fine under 68030 + 68040. But the MCL runtime was quite a bit weaker: cooperative multitasking, less CLOS and a less capable memory management.