Merckx (doper) won the Giro and Vuelta the year after and then the triple the year after that. I'd say it didn't hamper his physiology. Are you sure you don't mean that he said he'd never better his Hour performance, not that he was never the same physically?
4th Oct 18 - Mikkel Bjerg - 53.730 (not UCI sanctioned)
19 y.o and at sea level! :)