They can certainly put ABP out of business, which honestly is fine with me. But UO isn't a business AFAIK, it's a Free open-source project. That's like trying to put vim out of business; you just can't, because there's no business there. Google can certainly change Chrome to prevent UO from working, but they can't do the same for Firefox, nor can they really for Chromium (though their actions can make it difficult, but that could also mean that many other non-blocker extensions won't work if they're too restrictive).
Finally, these aren't the only browsers out there. Apple's Safari might also refuse to go along, and even go directly against Google somehow (including their own Google-ad-blocker, or helping UO work on their browser). Apple doesn't live on ads like Google does.
Finally, as for legislation, that doesn't make much sense either. First, I have a hard time seeing any Federal US laws actually succeed that force users to view ads, and even if such a law passed, there's no way to enforce it. How would they? Are websites going to sue users whose browsers don't actually download all their ads? That'll be great for PR. Or are they going to block users who don't download the ads (they're already doing this on some sites). If it gets that bad, it wouldn't be hard for ad-blockers to switch to simply downloading all the ads, and then sending them to /dev/null instead of displaying them.