Symbolics could have built computers with custom hardware. At some point in time the 64bit RISC CPUs (MIPS, Alpha, SPARC, POWER, x86-64, ...) were all powerful enough to run Lisp. Genera got ported to the DEC Alpha). They would have been also powerful enough to run Lisp as an OS. But that would also mean writing device drivers, interfaces to hardware and a lot of other stuff. It would have cost substantial amounts of money to port Genera to the metal. Who would have bought that? The idea of a high-level language running as an OS on the metal did not have any future. I don't think there are good examples where this was successful on the market.