The budget from advertisers is already credited to Google - if such tactic becomes popular, advertisers would get lower returns (bounces would be way higher than now), and move their budgets elsewhere, harming Google's business.
Here in the real world where that’s not gonna happen, they do benefit though. Adblockers getting popular makes sense, but some sort of ‘screw with google’ plugin (which already exists) is too niche.
Google doesn't bill the campaign upfront. It bills every time a threshold is crossed - for example, whenever you spend $500.
> move their budgets elsewhere
This assumes Google isn't effectively a monopoly. Is that assumption valid?