You can run 1960s System/360 binaries unmodified on modern z/OS. The system also uses a lot of "high level assembler" and "system provided assembly macros" making a complete architecture switch extremely painful and complicated.

They called their new architecture "ESAME" for a while for a pretty obvious reason.

I don't think that would change if the underlying architecture changes; IBM has been committed to backward compatibility for a long time. Some hypothetical future mainframe class IBM ARM would undoubtedly be able virtualize a 360/370/390 without breaking a sweat. And ARM will undoubtedly enable IBM to add custom emulation hardware to their spin on ARM if they need it.