Lance has said in the past that he thinks it's important for a Tour de France winner to have at least one stage win during that Tour. It's certainly possible to win the Tour without winning a stage, but to Lance that has been a mark of something less than greatness. Of course, in the past Lance has been able to win on at least one mountaintop finish - and this year he's running out of options.
Today's stage was really the last mountaintop finish of the Tour. There is one more (on Thursday), but it's after a not-very-long climb, which makes it harder for a select group to get away without the peloton being able to catch them. And it's not really a mountaintop finish, either, as there's a brief descent to the line. So at this point, it's looking more likely that Lance's only remaining opportunity for a stage win will be in the final individual time trial on Saturday, the day before the Tour ends in Paris.
If he gets to Paris without a stage win (it's possible - there are some great time triallists still left to challenge him), it will be evident that this was exactly the year to retire. A man clearly still at the top of his game, though not as dominant as in the past. As with nearly everything else in his life of late, this will have been very well calculated, indeed.