Should would could. Fortunately, in the real world laws sort out these things. Imagine that the former Looking Glass Studios reunite again, re-purchase the IP of the Thief series, and make a remake of the original game. Or... re-distribute the original game in any way. Not cool.
I don't see anything lost here anyway. You can still buy Thief Gold on GOG, Steam... wherever you want to. Holds true for most of the other "freely distributed" games there as well.
I bought Thief Gold and Thief 2 on Steam some time ago. They don't have any DRM, so, you can freely copy them from their folders elsewhere, patch them all the way you like, and they run perfectly. For 2 or 3 € i paid for them, i think it's pretty much a no-brainer to not go into the legal void, and, especially, pay the owners what's right.