Oh yeah, didnt know about the HP. Great feature.
For the sake of discussion, why not?
Of course, mint will bother those players who already have multiples 3/4x31 with various natures ^^. But they will thanks in the long term as metas changes builds will change, they can also use those pokes to keep refining for the 5/6x31. I dont think grinding should be the true endgame, anyway. Ranking and battling should be. So the earlier you get a team you feel comfortable with, without cutting off too much effort out of the ecuation if you want to get acces to various teams, the better.
It wont hurt the economy neither. As mints should be a goldsink that reduces inflation.