One assumption made by anyone that says learning on a P-47 is possible is that all or even most instructors, who might be excellent flyers and teachers, could fly a P-47, much less teach on one. Add in that the the "already built" plane might be on the heavy side, not built well or a difficult plane to fly. For a good instructor, saving a trainer plane on a bad approach that is about to stall or heading towards the pits upside down is no big deal. With a warbird there frequently is no "saving" once a mistake is made. On top of that, trainers don't sustain nearly as much damage in the same crash as a warbird would. Go watch the Top Gun vids, lots of the best builders and flyers around, lots of busted retracts and bad landings. You're right, trainer planes are not cool, but neither is crashing or giving up.