That a browser either maintain your own entire browser engine or you can't support additional types of extensions is a false dichotomy. Vivaldi chooses nto to support it just as much as the choose to integrate VPN, an email client, RSS, Atom, a completely custom UI, and probably other things I'm forgetting that Chromium doesn't ship for them.

What is changing is what Chromium will maintain, what Vivaldi decides to continue supporting is their own choice. My guess is it will be more random crap as well, but not because Chromium is/isn't supporting those things.