"There would be no gain from using ML in RDBMS query planning. It's regular data; the AI's trained model would just be a recapitulation of the query-planning algorithm we already have."

Most of what you wrote seems fine, until I got to this. A query optimizer seems like something that tends to be very opaque, very complex, and in my experience blows up without a good explanation frequently in typical situations. It's also based on a lack of complete data about the problem domain, to the point where an optimal algorithm seems hopeless. I'm not saying an AI approach automatically can be better, but at least you (I) can envision it being better, perhaps less brittle and taking into account dimensions and possibilities a human doesn't. And the non-AI solutions aren't trustworthy in terms of bounded quality so you have not got a lot to lose.