You can block the network calls to ad networks on the OS level by intercepting the DNS requests. That's a lot cruder and less flexible than the fine grained control over the DOM provided by a browser-based ad blocker though. If DNS level ad blockers became prevalent websites would also move to circumvent them, which would be a lot easier than with browser-based ad blockers.

It's better to just not let a browser that's user hostile enough to prevent ad blocking win a browser war.