The tricky part is that Google isn’t wrong about Manifest v3 increasing security for some people. Just allowing any extension to access the full URLs from a webpage is honestly pretty sketchy for most things that aren’t adblockers. Think about Beth in accounting who has 27 bloatware toolbar extensions installed on her home PC, which are happily collecting her full browser history and sending it off to gods know where. Manifest v3 is targeted at increasing security for those users, by making it more difficult for extensions to track you.
The issue is that it also makes ad blocking virtually impossible, because the blocker is forced to just trust that the browser is being truthful about what is and isn’t on the page. And when the browser (developed by one of the largest advertisers in the world) has a vested financial interest in displaying ads, there’s very little trust that the browser will actually be honest.
The issue is that there’s not some sort of “yes, I really want this extension to have full access” legacy workaround built in. Yes, it would inevitably be abused by those scummy extensions, which would just nag idiot users to allow them full access. And the idiot users, being idiots, would just do it without understanding the risks. Even if Chrome threw up all kinds of big red “hey make sure this extension actually needs full access and isn’t just tracking your shit” warning flags, there are still plenty of users who would happily give bloatware full access without reading any of the warnings. But it would also allow ad blockers to continue to function.