It's not just that. There is big difference in winning the match played to the best of 5 sets than playing best of 3 sets. If This match in the final was played with Masters rules Thiem would be winner. The big 3 are still strongest in the head, this goes double for Djokovic, he is undisputed champion in that category. That extra match length gives him more opportunity find cracks in his opponent game. Best from young generation have the weapons to hurt the big three, but they are still lacking a bit of consistency and experience.
Yeah, the longer the game goes, the changes of winning is higher for those guys. Because they can broke those guys mentally in a 5 setter, because they will have to elevate their game one notch and close out the shows. Just like what you says, once he sees an opening or weakness after 3 sets then the big 3 are going to exploit it to there advantage. Slowly these young generation will gain the experience, what I would like to see is for them to really develop their weapons, whatever it takes just to break Djokovic, Nadal and Federer.
The catch here is that if they are going to learn from playing against this tennis greats. We've seen that they can evenly played the first 3 sets even winning the majority of them. But when it crunch time, the big three knows to tap their inner strength and go on the next levels. And I don't think that the young generation will learn that from playing them, in my opinion.