Anyone with a mildly popular extension that has read/write * would be offered lots of money to sell it to, usually, scammers or hackers.

Maybe you're willing to pay the price, but that doesn't mean it was what's best for the ecosystem.

At the platform level, you have to have a security model, and sometimes it will conflict with functionality. I’m sure there’s a lot of potentially interesting browser extensions you could build with the ability to read and write arbitrary files, but Chrome has decided (much less controversially) that the sandbox is key to their security model and extensions can’t ask to escape it.

If manifest V3 ad blockers were nonfunctional to the point of being broken, I’d be more concerned, but in my experience they’re perfectly OK.

uBO Lite exists and I can't see any visible difference in how well it works. So, it's not a price you need to pay at all.

My content blocker on Safari blocks all the same ads as ublock origin in Chrome, with no supposedly no risk of outbound data.