Over time, things get old. It happens to everything - DOS was taken out by its shiny new big sister, Windows, and Windows NT was taken out by Windows 2000, and from there NT's upgrade of XP, XP to Vista, and so on.
Well, some programs don't take the aging process well. Sure, there's patches, clients, and other assorted whatnot, but over time, the developers just throw up their hands and say "forget this". Technology is too advanced, or maybe the developers just got bored. Or maybe it's planned obsolescence; if you're just patching your old software, you're not buying new stuff.
The program is now unsupported, there are no patches left to release, or they can't make any due to compatibility issues which mean even trying isn't feasible. The program is forgotten.
Welcome to the wasteland of Abandonware, where the forgotten languish in technology hell, since they can't keep up.
Websites such as Abandonia exist to allow people to play once again games that they used to have for their old 386 but have now slipped into the realms of abandonware. Many abandonware websites maintain a semblance of good faith by refusing to allow download of any game still being offered for sale. For example, 3D Realms used to offer all their old DOS titles (most notably Rise of the Triad) for sale from their website, and such they're very difficult to find on abandonware sites.
Philosophically, the "abandonware" argument is one that goes beyond the legal sphere into the realm of artistic morality: it is the assertion that a company that refuses or is unable to profit from a work for which it has gained the legal rights, is immorally acting to the detriment of art if they choose to sit on the property and allow no access to it from the world at large. The specific obsolescence problems with computer and video game technology have forced the issue: older books, movies and music generally don't become unprofitable from being behind a technological curve as games do, and if at all, nowhere near as quickly. The best movies, books and music from twenty years ago are easy to find and easy to buy, and easy to view, if perhaps in a different format than they once were. Not so much with games.
The rise of digital distribution has seen publishers put their entire back catalogue on services like Steam, and there are even online stores such as GOG.com who even going as far to develop their own upgrades to them so they'll work on modern computers. See also Emulation.
Home of the Underdogs was, for quite a long time, and now is again, an Abandonware 'museum', where users could find archived copies of many Abandonware and Freeware games. As the name implies, the website is (mostly) dedicated to underperforming, rare, or just plain bad games.
Macintosh users can find applications in the Macintosh Garden, once directly part of HOTUD but now just link to each other. In both HOTUD and Macintosh Garden, Sturgeon's Law takes effect, and how!
Emulation is a related topic, with related problems. The current owners of the Amiga brand, for example, have been keen to stress that the "Kickstart ROM" needed to boot an Amiga (or an Amiga emulator) is not abandonware, thank you very much. (Amstrad have said that the ZX Spectrum ROM, on the other hand, is perfectly acceptable for emulation purposes; and even if it wasn't, plenty of people have hacked their own version together after some crazy nutbags managed to document the function of each and every last one of the 16384 bytes that forms it, and patched it to deal with the (remarkably few) bugs.)
Compare Keep Circulating the Tapes.
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