From the subscriber agreement:
Steam and your Subscription(s) require the download and installation of Content and Services onto your computer. Valve hereby grants, and you accept, a non-exclusive license and right, to use the Content and Services for your personal, non-commercial use (except where commercial use is expressly allowed herein or in the applicable Subscription Terms). This license ends upon termination of (a) this Agreement or (b) a Subscription that includes the license. The Content and Services are licensed, not sold. Your license confers no title or ownership in the Content and Services. To make use of the Content and Services, you must have a Steam Account and you may be required to be running the Steam client and maintaining a connection to the Internet.
Valve may cancel your Account or any particular Subscription(s) at any time in the event that (a) Valve ceases providing such Subscriptions to similarly situated Subscribers generally, or (b) you breach any terms of this Agreement (including any Subscription Terms or Rules of Use). In the event that your Account or a particular Subscription is terminated or cancelled by Valve for a violation of this Agreement or improper or illegal activity, no refund, including of any Subscription fees or of any unused funds in your Steam Wallet, will be granted.
There is no provision in the agreement requiring Valve, or anyone else, to continue offering downloads indefinitely, and the agreement explicitly states that it is revocable by Valve at any time if they decide to stop offering a particular product, or in the unlikely event they decide to shut down Steam altogether.
For those unaware, the term "Subscription" in this context basically means a game that you have purchased; they characterize it as a "subscription" to emphasize the fact that you don't actually own it ("confers no title or ownership in the Content and Services").