I'd certainly pay a fixed price per month for unlimited searches. I think Google knows that won't work so they don't offer it. Or they couldn't ask the same price that they get selling your personal data?
They do offer Youtube Premium without ads but the search engine space is really different. Microtransactions are not here yet.
If they offered a premium "no ads" search option it would draw too much attention from the media/public to just how bad the situation currently is.
So, even if they could increase revenue per power user (RPPU?) it would do irreparable harm to their ad-supported model (for non-power users) in the long term.
They do offer Youtube Premium without ads but the search engine space is really different. Microtransactions are not here yet.