Just because there’s some plot point that needs explaining because times have changed doesn’t make a movie outdated to the point of being unenjoyable-- for example, in The Awful Truth, you could nullify a pending divorce, at least in New York, in the 1930s, and in enough other places in the US for general audiences to “get” what was going on, by “re-consummating” the marriage so to speak. Yes, if a couple who had filed for divorce, but who had not had the divorce finalized had sex again, the divorce was nullified. This is why Irene Dunne and Cary Grant go to such lengths not to be caught alone together, to the point of her once lying and saying that she is his sister. They are a couple with an impending divorce who are already seeing other people.
Now, Irene Dunne does make the situation pretty clear through some dialogue, such as saying that things will be easier when the divorce is finalized.
[Mild Spoiler] Knowing the law is really important at the end, when a clock is counting down to midnight, but nevermind.
Anyway, this is still an hilarious and very personable film to which people in 2017 can easily relate, obsolete law or not. Feelings about interpersonal relationships, and recognition of physical humor or good one-liners has not shifted paradigmatically, so the movie holds up.
Guess Who’s Coming to Dinner is stale, though, because paradigms around mixed-race relationships have shifted light years since the mid-sixties. Not only that, the target audience practically no longer exists. What happens is that white liberals who have always given money and lip-service to equality of race relations, and raised their daughter to be a “color-blind” liberal, and now she has brought home a black man and announced that she wants to marry him, and suddenly they feel all sorts of negative feelings they didn’t even know they harbored.
There might be people who have actually been through that, and now their grandchildren are in high school who might find it interesting, and there might be people who are biracial adults who never knew their liberal grandparents struggled with anything, who might have an emotional reaction to it, but all the liberals who were being invited to search their souls in 1967 are mostly no longer with us, or at least don’t need a movie to prod them to their real feelings about mixed race relationships.
So that’s the difference, IMO, between truly dated movies, and movies that are just old.