I think there's more involved than simply undoing a change. Apart from the obvious tedium of chasing upstream code changes, the extensions themselves have to be installed from a willing (and trusted) repository. The chrome web store can omit extensions that use an API in an unsupported way.

Brave already integrates ad-blocking at a lower level than the webRequest API, so this is less of an issue, but you sacrifice your choice in blocker.

https://brave.com/improved-ad-blocker-performance/

The recent Chromium’s Manifest v3 controversy around the overheads of the various extensions using the WebRequest API to inspect and potentially block undesired requests did not affect Brave as requests are processed natively, deep within the browser’s network stack.

If we're thinking of traditional ad blockers, I can see where you're coming from. But really, I don't think that's in danger. There will always be forks, or binary patches, not to mention router/DNS level solutions.

What I find much more scary is the prospect of the entire web stack becoming much more locked down as a consequence of both (1) ad blocker prevention and (2) the consolidated browser market. Think Encrypted Media Extensions, but for everything.

If there were ten browsers in active use today, getting them all to agree to something like this would be almost impossible. Unfortunately, there's only 2–3 browsers, and if Firefox loses any more marketshare...