If Google makes enough changes that eventually makes it impossible to merge upstream code while maintaining compatibility with the old APIs, then Brave and everyone else will either be stuck with old and unpatched Chromium versions or they'll have to waste resources to keep things working.

Not everyone has the resources to maintain their own Chromium fork and it's not easy to keep up with Google, Microsoft, Samsung, etc. Just look at Firefox.

That's not actually true. Brave's system uses a Chromium commit hash as a base, then applies patches on top of that. They'll have to manually move the old API implementation into patches and continue to maintain it there.

Source: I've collaborated with / worked on Brave in the past.

Won't this create a problem for them when it comes to the extension store since no new v2 extensions will be able to be deployed there even though other chromium based browsers support them?