Connor McDavid hahaha okay thats an exception. Barkov played 54 games at 18. Jack Hughes played 61, 56, and 49 games at 18, 19, 20. Morgan Reilly played 18 games at 17. Kirby Dach missed significant time and is now showing that he might get to his potential. Bowen Byram missed so many games but is still a stud. etc. etc. etc. There are endless examples. Games are not the end all be all of skill and physical development.
As a matter of fact, all NCAA players play half the amount of games as NHLers any way. Slaf played 39 games, Cooley 46. Sure its better quality ice time, but its not as if Slaf doesn't keep training while injured.