There are no servers that will be running when steam is gone...however unlikely if Steam goes out of business or whatever that's it. In order to install a game on Steam you have to authenticate with Steam in the first place anyways. Getting banned is also probably pretty hard to do, but you guys are missing the point...that's not even a concern with some other DD sites. I have my gog.com installers and if gog.com goes bankrupt or any other number of things it will have absolutely zero effect on my ability to install and play any of those games. It would require nothing from gog.com, and they could disappear tomorrow and I would be unaffected. The same is absolutely not true for Steam, and that is a fact.
Anyways I am almost 100% positive Valve never made that guarantee anyways (regarding keeping servers up). What I *think* you are referring to is a long time ago Valve said (and I have never actually seen this quote) they would release some kind of patch that would revert Steam to offline mode. Two problems with this though...first...this is nowhere in any of Steam's service agreements/ToS/EULA/ect so they have absolutely zero legal obligation to do so. In fact, legally according to Steam's ToS they actually say the opposite.
http://store.steampowered.com/subscriber_agreement/
C. NO GUARANTEES.
VALVE DOES NOT GUARANTEE CONTINUOUS, ERROR-FREE, VIRUS-FREE OR SECURE OPERATION AND ACCESS TO STEAM, THE SOFTWARE, YOUR ACCOUNT AND/OR YOUR SUBSCRIPTIONS(S).
Secondly, even if they did, that would only work on the games you currently have downloaded and loaded into Steam in the first place...once Valve is gone there would be zero way to download your games. Even if you had some backed up, there's no garuntee this magic patch (if they even release it) would be able to authenticate your games properly.
Everything is based on their good will, and there's absolutely nothing legally binding them to do so. If a company goes bankrupt, I'm not sure how much good will is to be expected of any company.
Like I said, I like Steam and I personally feel the risk is extremely low, and if it my games were going to go kaput 10 years from now I really wouldn't that heart broken (life goes on). The fact remains Steam does use heavy DRM, requires online authentication, and your games are indefinitely tied to an account you don't control. Some people find this unacceptable, and it's not an unreasonable position to have.
If you, like me, decide the risk is worth it...great! No need to ridicule and discount the folks that don't, however.