I am not a lawyer and frankly, have little use for them. I say this not in any way to damage Steam, but to give some bottom line power and peace of mind to those who fear Steams power over their accounts.
If Steam decides to ban you from their service, that is their call. But if they cancel their contract with you, that also cancels the bit that includes their right to "update" or modify data on your drive. In other words, they cant deprive you access to what you've already downloaded, only access to their authentication servers.
Likewise if Steam ever goes out of business (everything that has a beginning has an end, Neo) the auth servers will go down. You wont lose access to whats on your drive, but the game will refuse to play without authenticating with Steam. In theory, you have lost the ability to use the product you have purchased. In the real world, your solution is to make regular backup copies of the steamapps folder onto a separate drive, and after buying any game through Steam, google " fixed exe" and save a copy for future use should it become necessary. Obviously use of said file would be a breach of contract on your part, while your agreement with Steam remains active. Once it is cancelled, however, go nuts.