My greatest concern is how the ultimate goal of any video game company is to either make a game that sells, or one that attracts people enough to buy the next game. No matter how much work is put into a game to make it play better, there's still the overlying motivation to SELL. I'm worried that because there's no more reason to attract more players and make any money, there would be no reason to put effort into expanding TF2 more than it has been. So this would lead Valve to accelerate the story; revealing the mysterious Mann photo character, meet the pyro, the ultimate goal of the administrator and her ancestors... these will just be dumped out, last update, kk GG lets make another game.
Though maybe they will continue putting effort into TF2 because of profit from the Mann Co Store. Since items are now their only method of profit from TF2 anymore, I'm guessing that the huge surge of new players earns them more off the store than from selling copies of the game itself.