I was fine with the simple ads in the sidebar of the Google results of yore. It felt like a fair deal.

I think the problem is then you eventually get SEOification. I suppose you can just reset every now and again. Maybe like give a training program to get SEO people real jobs.

I haven't tried Kagi yet but I'm curious how does Kagi combat SEO spammers?

Extremely well! No idea how it's doing it in the general results, but they're orders of magnitude better than Google / Bing etc. Plus you get to fine tune results yourself, ie: downrank w3schools etc so the garbage sites are less likely to appear in your results. You can even exclude them completely.

A good strategy could be to rank on the number of ads ... more ads in the page, lower the rank.

They sort of do that. Next to link, you have information about how many ads were detected in the site, and sites with more ads are lower ranked. They also have an index of their own which only allows sites with few ad detections (they use number of blocked things by ublock, so it also counts tracking links etc)

Go try it. You get 100 free searches per month, or something like that.

I'm a happy customer.

Pretty well to be honest.