Steam is a bit different story though. Its business model is similar to iTunes/AppStore than those of the examples you presented. I doubt it will lose the competition among digital distribution shop because there are countless people who already purchased several to tens of games on Steam. They will not change the platform easily like how you would simply buy from different game developers, change web browser, use different web site etc. Also even if the company experiences a huge random inner problem that would drive them unable to support Steam, I'm very sure someone else would purchase the license and continue the support as there's a huge money/potential on it.
I agree that Steam's business model gives it an advantage over the examples that I listed. The risk is still there though. And for me, as a potential customer, the rights that the current Steam license grants me simply don't outweigh this risk.
An example (there are probably better ones, I'm just pulling this out of my head as I type): A couple of the biggest publishers could get together and decide that they can maximize their profits by establishing their own online distribution service, "Vapor". To "help" people over the threshold of signing up to yet another service, all their future games will be Vapor-only, and they also give out good deals to 3rd-party publishers as long as the games remain Vapor-exclusive. Since the founders of "Vapor" know that Steam has the advantage of an already existing customer and publisher base, they calculate with a period of deficit spending in which the main goal is to eat up as much of Steam's market share as possible. As a result, Vapor becomes more attractive for publishers and customers as Steam. Supporting their strategy, the owners of Vapor also attack Steam's policy of profiling and storing user data, which leads to lengthy lawsuits and some penalties for Steam in the European Union.
To defend against the new competitor, Steam plans to implement some innovative features into new games which unfortunately mostly fail. Steam now faces the problem that people keep using their old games on Steam (thereby causing maintenance costs and expenses), but rarely buy new ones, and publishers are also switching over to Vapor. After a while, Steam is piling up a bigger deficit than it can handle.
Steam ends up being sold to another company which, as a first step, introduces a monthly fee for Steam subscribers. Accounts who fail to pay the fee are deleted. This change is added to the Steam license. According to the license, this gives the customers the choice to either accept the change, or reject it and leave Steam. If you haven't logged into your Steam account for a while, tough luck. Alternatively, Steam's new owner might also come up with a somewhat sneakier way to reduce the expenses caused by people playing old games: They say that Steam will be "innovated" and switch to "Steam 2.0", which has some more features, but "unfortunately" support for many older games "which no one uses anyway" will have to be stopped. To sweeten the deal, customers with more than 10 of these older games can get a new game for free if they claim it within 30 days.
Now, you can say that the above scenario may be "unlikely". I'm not so sure about that, but it'd be a valid opinion, and time would have to tell who's right. What you can't say (imho) is that the above scenario is impossible. All these things have already happened in the world of business, and there's no reason why they shouldn't happen again. Steam's defenses and future plans may be strong enough to prevent it or they might not, we can't tell until it happens. In any case, the _most_ unlikely thing in the scenario above is probably the name that I gave Steam's hypothetical competitor.
Psy-
Point well taken, but Andoo makes an appropriate counter-point.
Plus, if you are worried about Steam shutting down, you can always backup your games onto your hard drive, then play in offline-mode.
I don't think they let you play the games indefinitely, I'm pretty sure Civ5 would demand a re-validation at least after a hardware change. If there was a possibility to make such a reusable offline "time capsule" of a game's installation, then indeed some of my concerns would be moot, as people like me could simply make offline backups of their games from time to time. It'd be a bit of a hassle (since the backup would have to be repeated after each patch), but doable. But I don't think Steam allows that to the extent you suggest (it's a bit of a catch-22, they can't allow such backups to be installed on computers with differing hardware without another validation, as it would undermine their viability as a DRM method).
edit: Hell, if Steam goes under and the EULA becomes a moot point, all of your backed-up games can probably be legally hacked into a working state much like they are currently illegally hacked. Just get Nemo and I to google it for you.
I don't think that'd be legal (at least not in my country). But to be honest, before I convince myself that I could use potentially illegal methods to overcome the disadvantages of a service, I'd rather vote with my wallet and not use the service in the first place.