Yes, the odds are against him. But let's work through the math.
Pettite picked up his sixth victory of the season tonight. He's averaged 15 wins per season over the last four seasons, and he's been consistent: 17-14-15-14, with at least 33 starts and 200 innings in each season.
Let's assume he wins nine more games this season. That would leave him with 230 wins at the end of his Age 37 season (he turns 37 next week, two weeks before the rough midpoint of the season) and thus 70 short of 300. Is winning 70 more games really so improbable?
Tom Glavine has won 54 games since his Age 37 season.
Randy Johnson has won 100 games since his Age 37 season.
Jamie Moyer has won 119 games since his Age 37 season.
OK, so Glavine and Johnson and Maddux are all going to the Hall of Fame. Moyer's not, but as data points go, he's sort of in an experiment of his own.
David Wells, though? Pettitte can't do what David Wells did?
If Pettitte averages 14 wins per season from 20010 through '14, he'll land on 300 exactly with his last win in his Age 42 season. Twenty or 30 years ago, this would have seemed almost unimaginable, as very few pitchers were able to pitch effectively into their early 40s.
Things are different now. There's no obvious reason why Pettitte can't still be pitching when he's 42.
Of course, to win 300 he'll probably have to pitch longer than that. All it takes is one missed half-season, and that 14-per-season pace gets tossed out the window. Pitching for some lesser team wouldn't help, either. But I suspect that if Pettitte wants to pitch for as long as someone will give him a uniform, in a few years he's going to be considered a serious candidate for 300.