There are literally thousands of talented athletes in every sport that are perpetually held back by poor fitness.
It's very interesting to see professional athletes which have, obviously, sport at the center of their lives, and still clearly not physically at their best. There have been enormous progress this past 20 years, but you still have several very high ranked people who could probably be better if they had better training planning.
I remember reading about Agassi's physical program which was prepared by Gil Reyes (who wasn't into tennis before being introduced to him) in Andre's (excellent) biography. Sometimes you just need to meet the right person and your training regimen can be tremendously enhanced.
To be fair, everyone isn't born the same, bodies are different, and including more fitness in an athlete program, who already has to recover from competition and training efforts, must be hard to do. I guess some pro athletes are also satisfied with their situation and don't necessarily strive to make even more sacrifices to be at the higher top there is (to me it's completely understandable).
Better fitness is not always the answer either. For instance Roger Rasheed is known for being a fitness freak, but none of his players (Hewitt, Monfils, Tsonga, Dimitrov) managed to win a very big title, even if they visibly improved their fitness under his lead.
for finals Bo5 is fine, especially if you have any of the big four playing each other but anything else? TV people would love to bring it down to Bo3.
the casual tennis watchers is not going to watch an entire 4 hour match between two people they barely know in the third round.
To me, the very idea of going best of three in slams, even for just a few matches, is an heresy. I hope it never changes. Many exceptionnal matches wouldn't have happened with that rule.