I mean it a bit broader than that, but you are on the right track. If one looks at who listens to a particular band from [pick any decade prior to this one], if it is pretty much just those who grew up with it who listen to it now, it has not stood the test of time, and is likely to be virtually completely ignored once those people die off. It does not matter how great people believe it to be who grew up with it; it is simply not timeless music if people of other generations have no interest in it. To give a specific example of a band that has (so far) stood the test of time, The Beatles spring to mind. They are even enjoyed by a significant number of people who were born after 1990. And they are enjoyed by many who liked them back in the 1960's. And people in between. Of course, not everyone likes them, but that is true of absolutely all musicians. In the case of The Beatles, much of their music has been played so much that some people are tired of the songs, which is understandable. But they keep getting new fans, and that is the point about being "timeless".
Basically, anything that is not "timeless" is disposable music. Even if some old farts still listen to it, that means nothing. That is just nostalgia. If anyone who is reading this is an old fart who loves some band that the younger generation "doesn't get", feel free to blame them for their "bad taste". But if it is music that does not touch anyone outside of the generation for whom it was initially popular, it isn't timeless and it is disposable, period. If you imagine that it isn't disposable, don't worry, your children will prove it is disposable when they throw it away after you die.