I'm going to be an idiot here and point out that personally I don't expect a $29 game to keep me entertained for months or years, Sure why not you ask? Well, it's mainly down to my expectations of games these days, When I buy a game and play it for more than 10 hours I consider it well worth the price.
Sure, almost everyone else wants months of entertainment from what is a budget priced game, maybe i'm just getting older and hopefully wiser
Still, nothing wrong with trying to keep it fresh at all, I just havent overplayed the game to an extent where I've got REALLY good at it, if I was brilliant and played it as much as half of you I'd be very happy to actually pay for an expansion or "fresh" content. Good on you Valve for supporting the game so much after release for no extra cost