Well, except it's not that simple at all. Even if they immediately discontinue all previous models and only introduce new architecture models in one go, you still have several years worth of computers sitting on x86/x64. They won't be able to get rid of it in the OS until at least the mid-2020s.

As an example with the PPC->x86 move, they released Rosetta in 2005, stopped pre-installing it in 2009, and only in 2011 did then completely remove the ability to install it. I foresee a similar experience here.

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