John Isner has finished every season since 2010 inside the top 20 in the world. While it doesn't seem like he has been around very long due to his days playing college tennis, he is nearing 30 and is now a well established veteran of the ATP Tour. He may not have turned into the savior of American tennis people were looking for, but he has been best the
US has to offer
for the last five years. American tennis
fans have started to move on to younger generations, but there is no reason to
believe Isner will be going anywhere in the next several years.
Perhaps the best player to turn to for purposes of analyzing Isner's future is his fellow big serving "giant", 6 foot 11 inch Ivo Karlovic. Karlovic is nearly 36 years old, and still going strong inside the top 30 in the world. Karlovic recently defeated world number one Novak Djokovic, showing age has little effect on his game. This isn't overly surprising, given that he was never known for his athleticism or scrambling ability. However, it still must be encouraging for Isner to see that Karlovic is able to compete with the best in the world well into his mid-thirties.
The key for Isner will be staying healthy and continuing to improve the weaker areas of his game, notably his backhand and return. A few minor tweaks could make a big difference, since any improvements in his return game would make him an extremely difficult out. Even if he just maintains the status quo, there's no reason he can't be a top 30 player into his mid to late thirties. Changes in court speed and string technology may have transformed the game to benefit defensive players, but there will always be a place for someone who can fire unreturnable serves at will.
Isner may want to consider employing a more aggressive style of play as he enters his thirties, as he has a tendency to be content to rally from the baseline. This style won't get any easier as he gets older, but it shouldn't be overly difficult for Isner to transition to a slightly more aggressive game. With the best serve in the world to get him out of trouble, Isner can afford to take some chances that other players can't.
The attention placed on Isner may diminish in the next several years as younger Americans (hopefully) start to rise up the rankings. However, barring injury and mental burnout, Isner could continue to post similar results for nearly another decade.