If Steam goes out of business, then we can expect these types of patches so we can still play our games. If Egosoft is able to do it, im sure some code savy gamer can make a patch too.
In most cases you'd probably have to redownload the whole game as a pirated version, not just a simple "unDRM patch". Hopefully you feel confident with the Russian (mafia) repackaged versions. Maybe it is from the unpatched retail version, missing all the later updates that came through Steam. Pirates just want recognition for the original 1.0 release, much less to provide continued support by providing cracked updates.
And some Steam games might just be so obscure that no one is arsed to crack it, or at least keep it continuously shared, for you. I think torrent sites are shock full of dead torrents with 0 seeders.
So yeah, you'll possibly be able to still play many of your games, but it can be quite a lot of hassle.
Most likely you would see something similar to gamecopyworld.com pop up. Secondly you would not have to re-download the game, the majority of steam versions just require a cracked exe, with 3-4 .dll files that emulate the background of steam. In fact two of those .dll's are the exact same for all games that use the cracked exe's.
Just like you see no-cd and cracked exe's for store bought games the past decade, you would see the same shit for steam games. Heck, some people already get those files just so they do not need to even have steam running while playing games they own.
The whole concept is similar to not having a cd to run. And the safety level is the same as the the no-cd cracks..Just have to trust your source.
For obscure games, go check out gamecopyworld and see the crap that has been cracked=) Now the bigger issue would be, should valve look to ever close steam (I highly doubt, its too much of a money maker), too play it safe I would back up every steam game on multiple hardrives for long term storage to keep those games=)
However, all the above is null and void...instead of cracking each steam game...most likely someone would develop a system to emulate steam so all games that require steam would play them...
meh...but steam is not going anywhere anytime soon=)