Jaime: Even if Steam goes out of business and Valve doesn't remove the DRM themselves, there'll surely be an unofficial crack out in no time. As far as I know, there may already be working ones out there.
I agree that there will be cracks in such a situation, as the demand for them will be very high. However, if I deemed piracy as acceptable, I wouldn't subscribe to Steam in the first place, would I? ;) That's the absurdity in it: You can get a pirated Steam game for free, knowing that they can't remove it from you later - or you can pay good money for giving up your customer rights and allowing Steam to remove your access to the game whenever they want. Personally, I prefer to stay legal _and_ have my games, which GOG thankfully lets me. :)
Side note: If a Steam crack came out after a Steam bankruptcy, I recommend to be _very_ careful with it. Having thousands of former Steam customers longing for access to their games is an open invitation to anyone who wants to inject malware into other people's systems. The demand for such a crack will be very high, and many players will be so relieved to regain access to their games that they won't check the crack thoroughly (provided that the malware inside is even visible to the antivirus programs).
Jaime: To me, the important question is how reliable the backup function really is. For example, like someone further up already mentioned, quite often you have to download a sizable portion of a game again after reinstalling from a backup.
I don't know for sure, but I think this depends on the specific DRM that Steam has applied to the game in question (there are gradual differences). It might work, or it might not, or it might work as long as you don't change your hardware, etc.
