Speaking as an historian, I think there are some valuable thoughts here. This thread evokes for me the foul cliche of undergrad history papers, that so-and-so was "ahead of their time." There is no such thing as being "ahead of one's time"; everyone lives in one's own time and is shaped by it. What that phrase REALLY means is that the writer finds something about the historical figure that they happen to like or admire, and that the writer is sufficiently ignorant of the figure's time that they can't understand how his time produced him. "People back then were stupid, this guy was smart, therefore this guy was ahead of his time, because he's smart like me!" - not realizing that the assumption of people back then being stupid is not exactly evidence of smarts on your part...
Likewise, dismissing something as "dated" is the flip side of that same presentist coin. "This was made in the past, I don't like it (now, in the present), therefore I dismiss it as relevant only to its own time." To be sure, there are some aspects of life where timeliness is the essence of the pursuit. Fashion, for instance; the whole game of being fashionable is being attuned to what people will admire NOW. You don't want to get caught wearing last year's look, as that means you "lose the game." You can say something similar in music, if you're talking about the popularity of music - you can say that the wave of popularity for 70s prog was specific to its time, and that popular tastes have changed, so that later bands doing similar things (like Spock's Beard or Porcupine Tree) don't reach the same level of popularity as Yes or ELP.
That doesn't amount to much as a critique of the music itself, though. I can understand a record producer not wanting to invest in an act because he doesn't think he can sell it, the music they make is the kind of thing that would have sold thirty years ago but not today. But that doesn't mean it's inferior music. It may still be vivid, expressive, evocative, creative, virtuosic.
And, of course, there's the phenomenon of the classic, the revival, the retro. The present constantly pillages the past for raw material, in every field of activity. When something that had been unfashionable becomes fashionable again, we then decree it "classic" or even "timeless." Of course, it isn't. Such pronouncements have nothing to do with any quality of the thing in question; they are purely statements about ourselves, that we find something about this old thing that we have decided to like. For now.