Left 4 Dead 2 isn't really the same thing. Firstly, because it's more than just a map pack. The argument that it should have been released with an add-on price rather than the full-game price can be had, but it's a separate issue. Secondly, they didn't abandon L4D1. They still gave it free content releases, and I believe more are in the works for both L4D1 and 2. I understand not everyone can afford to support their games to the level that Valve does, but maps are too cheap to make to justify charging for alone.