Part of this hypothetical longevity comes down to the game's simplicity - I could explain the rules well enough by the end of this post that you could theoretically reverse engineer the bulk of the game - but it's not just that. You could get that in a game that was merely very short. Microscope has that special kind of simplicity where it completely occupies an elemental niche. It has exactly enough rules to be a cooperative storytelling game about the grand scope of history, and I'm not sure you could possibly improve it by either adding a rule or taking one away.
The closest comparison I can come up with is Tetris. It's a game that somehow finds the bones of the medium. It's what would be left behind if you started with an average game and took away all the parts that were not purely a game. It isn't necessarily an exemplar of what gamers like about gaming, but it is an exemplar of what makes non-gamers into gamers. Most of the books I read are potential substitutes for Dungeons and Dragons. This book is a potential substitute for charades.
At it's most basic, Microscope is an improv exercise. Its two most important rules are "don't contradict anything that's already been established" and "only the person whose turn it is gets to talk (well, talk, officially, in a way that's relevant to the game - it's not draconian about silence or anything)." Aside from those, every other rule is geared towards getting people to improv about the same thing.
The course of the game involves going around the group filling in a timeline, starting with the most abstract level - Periods, which are sections of history that span years or decades, then moving in to Events, which are things that happen over the course of days or weeks, and finally Scenes, where the players get down and freeform roleplay about a particular historical question (i.e. "why did the count betray the king?") Each one of these things is established by a player telling a short little story, and then a summary is written down on an index card - Periods go right to left, Events go from top to bottom underneath their overarching Period, and then Scenes are stacked vertically underneath their relevant Event. You don't have to play in chronological order, and each player can explore whatever they're most interested in.
The beauty of Microscope is that you can play it as a party game, you can play it as a world-building exercise, and you can even play it as a regular rpg. It's a short, 70 page book that, if there was any justice in the world, would be a family game night staple. Get that Monopoly shit out of my face, we're playing Microscope!
I actually feel a little embarrassed by how effusive my praise has been so far, so I want to be clear - it's the best designed game on my list (as far as I know so far), but what I mean by that is that when it's your birthday and your mother wants you to come over and play one of those roleplaying games you're always talking about, and your brother is in town and he's never roleplayed in his life, and you have to contemplate a single game you can explain, start, and get through in a single night - Microscope is the choice you can make and not feel like you're compromising. However, it's not the game you play every week for years. It's not the game you go on the forums and argue about. It's not the game you're obsessed with. You put it on the shelf next to Yahtzee and break it out in case of emergencies, but if you've got the luxury of playing something more complex, you're probably going to get more out of that.
Ukss Contribution: Not much setting here, but it did have plenty of examples. I liked the period of history where the corrupt emperor was persecuting martial arts schools. It's clearly inspired by the history of Shaolin, but I've always found that a fascinating dynamic in fiction - a centralizing state attempting to secure a monopoly on violence that runs up against an important cultural tradition, and thus it's an attempt to seek political legitimacy that undermines its own legitimacy. I'm not sure I'll be as openly analogous to real history, though.