Even though there is a possibility for me to not exist in 20 years from now, i bet all my money that the same games that are good old for today's standards, are going to be in the future, too. Plus a select few new ones from now, that were unexpectedly and exceptionally good. Like the Witcher series. For many reasons...
One of them, the most important, is that certain games, or certain game GENRES, have totally ceased to exist and no one makes even one indie game out of them. Like 2D, 2.5D games, arcades, platforms, etc. The past was full of them, and many of unparalleled quality, too.
The closest behind reason, is that certain games, are still top dog in ratings, even like 10-20 years from having been released. It is pretty hard for ANY newcomer to topple them, anyway. Games like Tetris, Castlevania: Symphony of the Night, Zelda, Sonic, Mario, are staying right where they are, regardless of what is released over the next many years.
Undisputed grandfathers and kings of genres. Thief for stealth and first person looter, diablo for action rpgs and hack&slash, etc... Not only they had been the first entries in their respective fields, but everything after them is called a clone, a wannabe, etc. And many people, especially the old ones, agree, that the newer entries in the series have shamed and disgraced them (diablo 3, Thi4f, ewww!)...
Remakes and Remakes. Proves just that. Things are stale, inspiration stagnant, ideas exhausted, everything tried and experimented upon already, since long ago. It's funny how people find the originals better than remakes, or the remakes to lack content that the originals did have. Or that the nice and shiny graphics are not enough to buyout or convert a player's mind, soul and opinion, sometimes.
Nostalgia debunked. When a new player is curious enough to sink in emulation, when a new player plays a good old game and ranks it higher than modern ones, it means the game is so good, that nostalgia becomes irrelevant, especially if he/she had not been around yet, when the game played and enjoyed was out.
That's how i feel at least. In 20 years from now, all time classics lists, highest rated games list and stuff, are going to remain the same as is today, plus a select few exceptions, of great new games. And unfortunately, there are not many of those around. New and good, is a rare combination, alas... They simply don't make them as they used to.