He came out in his first match at a tournament around 10 times or so, no way you can blame physical game for that, he's 19 not 39
That isn't the point being made. Sure playing a large number of matches is exhausting, but the traveling is almost just as bad. You have additional stress to get to a different location, then you have to adjust, get situated and then play your matches.
Kei had already won a title at 17 and made 4th round of a slam
Tomic made QF at Wimbledon last year and had a chance to perhaps knock out Djokovic. Novak was never really comfortable in that match, and it probably should have gone to at least a 5th set.
It isn't that I particularly like Tomic or any of the youngsters, but I certainly feel for them, and especially they have the pressure from the casual tennis world to be the next Federer/Nadal which is most likely not going to happen, at least for quite a while. The standards they put should be left out for comparisons. No, they aren't producing Hewitt or Chang-like numbers at a young age, but the game is different today, a lot more physical and mental, they need to develop physically to compete at the highest level.