So, in short, the issue is that Steam apparently has the right to remove content from their clients' libraries.
The first is reasonable, the second is iffy and would more likely be a technical limitation than a explicit choice on Valve's part, the third scenario is the problematic one.
As an example, Counter Strike: Source stored the servers in a human readable file and you could add alternative servers in the config file without a problem.
Nothing ruins future sales like lack of demand due to past sales.
That said, it's done all the time via apt-get or windows update or other program updates. People think that this rises to a new level though, and given the fact that Steam users are younger, tech-saavy, and more vocal it's no wonder that it's raising a stink.
In addition, the "Challenge" add-on was given free to those who bought the original game.
This is a complete non-issue as far as Steam is concerned (the complaint about publishers shutting down multi-player servers is certainly valid). There is nothing that you could do with the game even if you still had it in your Steam library.