I find it hard to see how they could protect content from ad blockers without also crippling pages that self modify their own content. Perhaps they could put headers akin to content security policy that forbids external modification. Assuming a browser were to honour that header I could see bad publicity and a lot of people just moving to another browser which doesn't. Additionally, ad blockers aren't the only things that modify pages - breaking accessibility add ons could be more negative publicity (just like with Reddit).

I think browsers would be best off to let websites develop countermeasures if they're so sore about ad blockers. Perhaps they could use "self healing" Javascript libraries that put back content which is removed. Or they could just refuse to work if they detect an ad blocker, e.g. they stick some canaries in the DOM or along blocked paths to see if an ad blocker is present.