I see this bandied about often as some kind of reason L4D2 would be better...which is just completely wrong. Valve didn't buy L4D, they bought Turtle Rock Studios in its entirety, and the Turtle Rock people finished up the game after being bought/renamed to Valve South. Those same people also made L4D2. It's still a bunch of not-Valvers in all but name. It's just a different studio mindset.
Well regardless of semantics, I care less than nothing about either L4D1 or 2. TF2 has them both by the balls in terms of gameplay and staying power. Supposedly, they had a bunch of the TF2 staff onboard with the L4D2 stuff leading up to its release. There has never been a game quite like TF2, especially in terms of support from its developer. Valve has created a winning formula, so there's no reason why they shouldn't keep doing the things they've been doing.