If anything, Kagi has proven that Googles "SEO ruined it" argument isn't valid.

In the beginning, Kagi was mostly just API calls to Google and Bing AFAIK. Their results were still better for some reason, probably because they didn't have to consider how valuable the ads on garbage site were before throwing the site out of my results.

I agree with you to some extent. For example, there's no reason I can think of why Google couldn't allow user selected domain blacklisting or custom bangs.

But I fear that if a large search engine copied some of Kagi's filters/lenses for non-commercial blogs & forums then SEO agencies would dedicate immense resources to polluting them.

Not possible by definition. Kagi Small Web is non commercial sites only, by definition. And the list is fixed (and open source). There is nothing SEO can do to infiltrate it.

It doesn't have to be big if it's rich.

People will target Kagi because it's packed to the brim with people who have $5-10/month to spend on a search engine, and likely have $5-10/month to spend on other things too.

Why would they? The same manipulative techniques that work in my niche on Google seem to work even better on Kagi