To the extent that HTTP/3 is faster/cheaper/better than its predecessors, it should allow ads to be served more cheaply. Combine that with support in the dominant browser (and why wouldn't they, after all they did basically invent it and they have a vested interest in serving ads efficiently) and I'd say HTTP/3 has a pretty good shot at success.
> To the extent that HTTP/3 is faster/cheaper/better
This is rather questionable assumption. For almost everyone in the world it's not faster/cheaper/better than HTTP/1.1, definitely not worthy enough to even bother with it. So, the only way it can get anywhere is if Google abuses its position and forces everyone to adopt it. Which they probably won't do for such a silly thing, they get more out of it by coming up with more useless mediocre "faster/cheaper/better" protocols HTTP/[4567], because this pressures competition to waste resources on that, instead of on something that can compete with Google.