> and if progress isn't as fast as the planner expects, feed current progress info back to the planner (pages scanned, tuples matching), and replan with that new data.

That would require keeping track of those stats in every query execution. That has a price that may or may not be worth it.

And yes, you could make that behavior an option, but, for better or for worse, PostgreSQL tends to be opposed to having queries indicate how it should do its work.