We use the term “dated” as a sort of declaration of mediocrity or lack of merit but how can it be? Are we confused about when Beethoven’s music was written? Does it’s era make it redundant? Why do we think that music that belongs to the past is somehow inferior because we notice how old it is?
If only modern music has merit then we are all doomed eventually to be irrelevant.
We *are* all doomed eventually to be irrelevant and/or nonexistent. Beethoven had many thousands of contemporaries you'll never hear about. You can be a Ph.D.-holding musicologist and still only be reasonably familiar with the work of a handful of those unsung contemporaries, which probably only happened because you were looking for a research specialty to tie your tenure binder together. Whether or not any of their work was more interesting or potentially could have been more important than Beethoven's in other circumstances, we'll never know. At some point we'll stop hearing about Beethoven too, or there will be no one left to talk about him.
And, sure, Beethoven was insanely influential on the next hundred-plus years of handfuls of dead white guys we still talk about, as cited by said dead white guys while they were still alive. But musicians-- of all eras-- typically only become successful and/or possibly remembered through the combination of unbelievable talent, unbelievable effort, and-- sadly, here's the most important part-- unbelievable luck. (That luck can also include being born a white male who somehow found themselves with enough direct access to high society to attain publishing and/or performance.)
When I personally hear something as "dated," it doesn't always mean it's mediocre and/or without merits I can appreciate. It means it creeps me out to some degree or has some component that either makes me uncomfortable in the same sort of way that facing death makes one uncomfortable or just outright makes me not want to deal with it and, yeah, hell, I don't know why.
Usually it seems this is a facet or aspect of the recording technology involved or the production. Yes, I can tell something by Bach is "dated" because nobody has really written in that style for centuries (word in my music history courses was that he was considered a throwback in his own time). But it's not nearly the same as being super-creeped-out by 10CC's "I'm Not In Love," which I always am. Doesn't matter how fascinating the story of that vocal choir played via mixing board might be (it's totally fascinating). Doesn't matter if the underlying composition is reasonably sound (it is). It just sounds icky to me in a way that makes me very uncomfortable, and when I hear it, I immediately want it to stop.
It's not a rational thing. Enjoyment of music is never anywhere near rational or explainable. As a species we enjoy music, but what music each of us can enjoy or not enjoy is tied up in all kinds of baggage, both visible and invisible, in our upbringing and personal experiences.
As for prog, yeah, as much as love the idea and love so much of the music, a lot of it still sounds really dated to me. ELP is fun sometimes but still sounds totally ridiculous. _Lamb Lies Down_ sounds super-ahead of its time, but the same can't be said for earlier Genesis. There are a lot of lesser-known prog bands whose work hasn't aged well at all.
Mainstream-prog-wise, I have never enjoyed listening to Rush. I don't know if I'd call Zappa prog despite his reference in thread. I adored him in theory growing up. Some of his 60s / early 70s stuff has held up, some of it sounds incredibly dated, and almost all of his mid-70s and later stuff sounds MUCH more absurdly dated and I really don't need to hear it ever again.
There's an episode in the 3rd season of the underrated Adult Swim series _Eagleheart_ that spends some time poking fun at prog rock values, which I suspect was written by people who actually really enjoy prog rock. They poke particular fun at the longwindedness and self-indulgence of the genre. We're certainly not at a time in Western culture that tolerates longwindedness. I probably should have stopped this post about four paragraphs ago.