If you examine what you're saying here and slippery slope it a little, examining past effort for misses and correcting them at scale is not worth doing? There are many ways that the second part of your perspective are incorrect (burden / lack of point). I bet that a coding agent could in an automated fasion find at least reasonable additions that you'd say add value and reduce potential for error long term that you'd find valuable. They've probably already done so (I don't know postgres dev at all, so just supposition here. They will 100% do so in the future.

I think it's rather unfortunate that writing regression tests is seen as a heavy burden. Compared to determining the cause of a bug and the right strategy to fixing it in a maintainable way, writing such a test should be simple, no?

I'm not at ease regarding LLM generated code changes to a project with a (hopefully) long expected life time, but LLM generated regression tests should be less contentious. I wouldn't expect them to be maintained much; rather, if they don't perform as intended against a future build, just have them recreated.