How common, I wonder, if well maintained (but high mileage)?  What is the criteria to determine if replacement is indeed warranted?  If 50s era suspension Ford/T-Bird ball joints last nearly forever (when kept greased), shouldn’t the drag link ball stud also?  

6 VOLTS/POS. GRD. NW INDIANA

If it's adjusted as tight as you can get it and it's still loose check to see if the stud is egg shaped, in which case it should be replaced. If properly maintained they should last about as long as the suspension ball joints. The felt seal around the shank of the stud is important. 

Drag-link rebuild kits are available. I rebuilt the one on my 55 Tbird, but I can't say if it was really needed.