Movie studios and music publishers have long realised that a back catalogue of good titles is a valuable asset that if well managed can continue to generate strong revenues for many decades. It is not at all obvious that game publishers care as much about the value of their older games. Everything is focussed on achieving maximum revenues in the first few weeks after release. If a game is lucky it will get tossed over the wall to one of the budget labels who will extend the availability of the game for a few years but even then the obscene haste with which the original publishers wash their hands of a title is remarkable.
On a positive note the advent of digital distribution is a major positive step and has made a major impact on the availability of older titles. I have noticed that online sales of an older game on Steam for example do spark a minor flurry of interest on forums and websites relating to the game.
Newer drm schemes which require online connection even for single player games are a worrying development however. Once the servers shut down (and they will) the game dies forever. I guess far sighted developer will include some form of bypass switch which disable that drm once the game shuts down but I would not have confidence that this will always happen. Generally once a game is dropped it is dropped like a stone and even once more patch is seen as a bridge too far.