They've made money when they first came out, and are still making some. Chances are that one hundred years from now, they'll still be as popular, albeit possibly slightly revamped. Look at anything out there, and you can say the same about them. Cars, homes, money, sports cards, comic books, novels, and even telephones. This is why a 1957 Chevy is worth so much.
Classics never die.
They'll be here long after we're all dead, and they'll still be classics when your grandchildren become grandparents themselves. The same could be said about computer software. If not for the first pieces of software that came out, many games today probably wouldn't be here. For instance, when Dungeon Master came out, it became a hit immediately. When I bought it, I remember playing days on end till I grew tired, only to return later.