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.
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.