pretty much there is no data that would really prove or disprove your point. Basically for position players one study says that 29 is on average when the decline starts. Injuries, play a huge part in the start of the phase. Fielder' neck is the freak injury that derailed his career. Others have had an injury that starts a decline. There is no doubt in my mind that training and lifestyle play a huge role. You can look at other sports and see that. Then there are the genetic freaks. Pucket didn't slow down until the eye. They test for steroids. Athletes learn to beat the system. How much o that is going on I do not know. They do help prolong a career. Machado could be great for another 8 years like Beltre, be like say McCutchen and be merely very good. . Or be like Heyward and forget something after they ink the contract. The odds are likely that he would remain a 30+ hr throughout a 7 year contract. He could stick at short for 2-3 years. What I don't know is what is his drive to be great is, or the passion for the game. If money has been the goal all along, what happens once he gets it?