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.
The very last version of the Symbolics machine wasn't hardware but instead was a virtual machine running on the Alpha processor. So I would say it was a road that was taken, though it was not a success (probably bad timing since I agree it was a good idea).
A unikernel like MirageOS is basically an OCaml Machine, except that you don't develop software within it. Someone could create a Lisp on Xen equivalent.