Aside: Apple's 68k-on-PowerPC emulator was an amazing piece of work. It didn't run 68k code in a separate environment; it allowed 68k code to run alongside PowerPC code, to nearly the same level of transparency. Applications could mix ISAs in libraries and code resources, and system extensions could patch PowerPC system functions using 68k code or vice versa. It was surprisingly efficient, too: the first PowerPC systems used system software consisting mostly of 68k code, and still ran faster than the native 68k systems they replaced.

I've never seen anything quite like it since. About the closest I've seen is the way that Thumb code can be integrated with native ARM code, and that's explicitly just a different instruction encoding, rather than a separate ISA.

The problem with that, at least in the case of Rosetta, is that the QuickTransit technology underlying it got bought by IBM and disappeared from the open market, so doing that trick again would be...a moderate time+cost sink.

Wiki for QuickTransit seems to think that a number of prominent people from Transitive hopped to ARM and Apple (which might be telling, given the claims of the original post), but has no citations.