The reason Valve gets away with any issues with Steam is their reputation for being nice. If Valve stated they could steal your relative's fetuses in the user agreement, the fuss wouldn't be too big because...it's Valve right? They wouldn't do a thing like that!

Such a stance is partially justified, due to Valve being privately owned by a very small number of individuals and their completely flat corporate structure. Gabe Newell has announced that if Valve ever went bust they'd patch away all the online requirements, ergo it will happen in the minds of most.

The actual success of Steam can be put down to two factors:

1) Convenience: especially for older games, patch-hunting is boring.
2) Price: Steam regularly runs quick deals on a given product or range, which are liable to knock 50% or more off prices that weren't too bad to start with. Paying £3 for an AAA game that's only a couple of years old is pretty good value.
3) Social/additional functionality: I run even non-Steam games through Steam, just to get the overlay chat and web browser.

Steam is technically DRM, but it's pretty innocuous. You have to log in to Steam to start a game: no Windows processes/rootkits/authentication keys required. Most people are quite happy to accept that for the service provided. If I want to play a game, I run Steam, not because I have to, but because I want to.

In the unlikely event of Steam disappearing suddenly (despite it basically being a license to print money), I wouldn't feel at all guilty about simply adding the Steam equivalent of "No-CD" patches to my games, which wouldn't be very difficult anyway.