But you do not need to discuss that. Rather suppose ad-blockers are working less effectively under v3, leading to additional ads being shown. That would increase the attack surface.

If ad-blockers really will be less effective under v3 remains to be seen. But do you think it is an unreasonable perspective, given the technical limitation?

Given that the reasons behind Google breaking backward compatibility are obviously primarily because they want to weaken ad blocking, I think it's perfectly reasonable to talk about this.