[ OK ~ now, the hard part. Rare earths aren’t all that rare. Actually they are common as dirt ~ the problem arises in their origin ~ which is almost always in rocks containing concentrations of THORIUM, and thorium is a tad radioactive and as it sits there for billions of years fissiling away, it yields up certain radioactive materials that need to be REMOVED during the processing of the ore with the rare earths. ]
If we were burning Thorium the problem would solve itself.
You’d still have the problem of eliminating the radioactive byproducts SAFELY. That doesn’t go away. Ultimately you can see the makings of a vast surplus of rare earth minerals however.