If two individuals come to an agreement outside of google why can't they swap or trade? I can think of many possible reasons BTW but I am interested which ones were the criterion, not just side effects good to have.

The model proposed demands the old address become an alias to the new address. This seems like a sensible simplifying decision.

I may be wrong, but I believe if you formally repudiate an identity in Google it's blocked for reuse, forever.

GSuite may be different.