One of the "benefits" to Microsoft of using Chromium is reduced development costs and they're not going to get that if they let the forks diverge too much.
Right, but 'we don't support this!' would be a great look when most technical people are already strongly opposed to Manifest v3. I'd call it an easy win, but of course they'd have to maintain that and possibly implement / design their own API's when Manifest v4, for example, comes out... so it's definitely not as easy as it may seem.
I'm sure Microsoft wouldn't mind spending more on development if it means market share from Google if Chrome breaks everyone's extensions and they still work on Edge
