So, as the title suggests, you should probably read this post before continuing.
First, after reading the excellent comments after the previous post, I'm slowly realizing that my original question, which I thought of as two sides to the same coin, is actually two distinct questions. Those questions are:
1) Are there a group of songs that, for whatever reason, would be hit songs at any point in time?
This is the Johnny Cash and Co. listening to Journey part of the question. Asking "Would they think that this cover band was the absolute greatest band in the history of the world?" made it sound like a kind of personal preference question, like I was really asking you to guess at what kind of music 1950s-era Johnny Cash was into. A better question would have been: Upon hearing the cover band, would the Sun Studios guys have immediately thought, "That song is going to be a huge hit. It's going to sell fifty million records. I wonder if I could steal it and record it myself."
This question isn't so much about the development of culture as it is the quest for the perfect pop song. Basically, the question becomes, "Are we going somewhere with music? Is perfection attainable? Are there songs that the majority of the populace would enjoy at any point in history, and can we identify those songs? Are we getting closer creating the perfect pop song, or at least identifying the elements that make up a perfect pop song?"
Along these lines, I'm fairly sure that "Hey Ya!" by Outkast would have been a hit at any point after about 1964. Is that really true? Are there other songs like that?
2) Would I, today, enjoy the same songs that my future self, 20 years into the future, enjoys?
This one, I think, was completely understandable in the original post, and the fact that two thoughtful commenters came to opposite conclusions sort of proves how much fun the question is.
I'd also like to address a question from the comments, one by a roommate of mine who has an excellent movie blog you all should visit. He asked:
here's a question for you though. there are a billion genres now that completely didn't exist 20 years ago. for instance, i'm pretty sure there wasn't emo in 1988. i could be wrong, but who knows. so my question is what kind of genres will exist 20 years from now? will there be so many different varieties that the word genre ceases to have any meaning? what separates acid emo punk from acid emo speed death metal? and the big "if" is will those people still want to cut themselves...
Honestly, I don't really pay that much attention to genre, as I think it's mostly an exercise in boring people making themselves sound interesting or, conversely, a way for music reviewers to sound smart while saying the same things that music reviewers have said since the dawn of time. Maybe there wasn't emo as emo in 1988, but there was soft, minor-key music about loneliness and alienation (as a point of reference, The Cure's excellent Disintegration came out in 1989), so who could really tell the difference ... except for the people whose job it is to create the difference. This is why you can almost never find a starting point for the existence of a genre. Punk rock started with the Sex Pistols in 1977. Unless it started before that, with Iggy Pop. Unless it started before that, with the Velvet Underground (Fun Fact: Every genre in the history of the world started with the Velvet Underground. At least according to hipsters. Ask them to explain how the Velvet Underground invented hip hop). Unless it started before that, with, I don't know, virtually anyone affiliated with the Nuggets box set. Unless it started before that, with a young Neanderthal boy smacking a rock against his cave wall and screaming because he's mad at "society."
Just to prove your point, I assume you made up the genres "acid emo punk" and "acid emo speed death metal," but I could probably go out and find bands that fit those descriptions (they would probably write songs about sad, skateboarding dragons). Does that mean that those genres exist now? Of course not. We just created them.
I think you could make a pretty credible argument right now that the word "genre" has ceased to have any meaning. Here's a question along those lines:
Will you, in 20 years time, like the same genre of music that you do right now?
Here I'm not talking about any kind of personal evolution on your part. You could have the exact same taste that you do today, but the idea of genres will change right under your feet. Here's an example:
If you liked the Beatles in 1967, you liked rock and roll. There was no doubt about it. Today, on the other hand, think about how far those terms have become separated. Rock and roll is a much darker, angrier thing today. The top three rock albums on iTunes right now are Nickelback, Guns N Roses, and Metallica. Would a Beatles fan in 1967 like any of those albums? I mean, maybe - he could be really open-minded about music. But I certainly wouldn't attempt to infer any correlation from the relationship. If the Beatles released an album today, it would not be considered rock and roll by anyone except old people.
This is somewhat strange because Beatles-esque music is still being made, but today we'd call it Britpop, or power pop, or something with the prefix "indie." Cheerful, driving pop music with intricate vocal harmonies still exists, but it's not rock and roll anymore. You could listen to the same music for years, but, if you believe in the existence of genre, you're listening to something completely different now.
For another example, albeit in reverse, think about Metallica. When the Black Album came out in 1991, they were a heavy metal band. They were not rock and roll - in fact, you could argue that they were a reaction to rock and roll, a rejection of the coked-out, big hair and spandex caricature of the "rock star" that predominated at the time. Metallica was music for people who got sick of rock and roll. And now, without changing too much in the way of sound (I mean, they're not nearly as good as they were back then, they're old and boring and more concerned about suing their fans for copyright infringement now, but they make pretty much the same kind of music), they are now the definition of mainstream rock and roll.
So, to answer your question ... I don't think genre is really a thing. Was that your question?