edit: Steam adoption+in-app purchases. Good call.
It's a gutsy move, to be sure.
That being said. The advantage of making TF2 free-to-play are huge. Firstly, the hat-conomy generates plenty of revenue on its own. More importantly, TF2 could quite easily become a gateway drug for casual PC gamers to use steam. More users playing TF2 means more users looking at, say, Terraria for 10 bucks, or Left 4 Dead 2 for 20, or some other relatively recent game at a ridiculous price during a sale.
I really doubt many people are buying TF2 by itself anymore. Anyone who hasn't played it will almost definitely pick it up as part of a bundle.
I would also guess that Valve has something big up their sleeve to be willing to give away such an important money maker, but I have no backing for that other than instinct. Now what would be really awesome would be open-sourcing TF2.
Regardless, as people said the sales of TF2 have probably mostly dropped off, distributing the game for free costs nothing (well, it costs hosting and bandwidth but I don't suspect those costs make up any significant percentage of Steam's total costs, so effectively the hosting is free) so they have barely any income to lose and a massive fanbase and ingame transaction economy to gain.
Since they opened the in-app store a few months ago, they've made tons of money. TF2 now sells individual hats for up to $13, more than the retail price of TF2 during many periods. At this point, it becomes obvious: expand the market by making it free to play.
I love the robustness of TF2, and Valve has been awesome about continuously providing new content for free, but, it's something to consider.