I mean they have to keep the old elements alive because that is the data they use to render to the DOM.

What React could do is to catch that there have been changes made to the structure of the DOM by someone other than them and then re-render the full page. Which would probably not be the most performant solution for anyone.

But anyway then people would complain that React was breaking Google Translate.

Essentially you have two applications fighting to control rendering of the application state of one of them.