The technology to do this is already there. Apple will likely extend some piece of open-source work or acquire a small company which specializes in emulating x86 on whatever architecture they transition to. Emulators are complicated pieces of software, but they are well understood ones. We know how to make them reasonably performant, and Apple has had around four major architecture changes under its belt from which it can draw experience.

The existence of Rosetta does not prove the transition to be 'complicated'. It just shows the transition was a thing