I think the most surprising thing I noticed rewatching it last year was that Bill and Ted aren't actually all that dumb. They're not really stupid, they're just uninterested. They know who Napoleon is and that he's related to something called "Waterloo", they know Socrates is spelled differently than it's pronounced, they know at least rudimentary information about most of the historical figures judging by their presentation, and Bill knows what an Oedipal complex is. They just don't care about history all that much, and are focused on music, even if they don't quite know how to get off the ground there, either.
I think Bill and Ted endure because they're not idiots or assholes like they would be if created today. Notice that they're unfailingly polite to just about everyone they meet, and extremely sensitive to how others want to be seen or treated (despite how silly it is that Missy wants to be called "mom" by someone three years younger than her, Bill does try to remember to do it because he knows it's important to her), which is actually just them living their own philosophy of "Be excellent to each other." They aren't jerks to the history teacher who keeps them after class, either, not even behind his back. The only time either of them gets legitimately angry at anyone is when Bill thinks the "Medieval dickweed" has killed Ted. They're ignorant-but-not-stupid slackers who are at heart genuinely kind people, and that wins us over to their side instantly. The sequel forgets this a bit, and doesn't seem to like them very much on top of that, but I'm hoping the much-talked-about third film will bring things back around.
