Whenever I see discussion on classical music, it's a fun mental game for me to imagine it's a forum for metal instead, and these (assumingly) well-to-do academics are all metalheads who have a strangely archaic way of describing the specific merits of songs.
I don't think that's much of a concern in practice. We celebrate lots of works that people didn't find particularly noteworthy at the time but were appreciated at a later age (Bach, for example, was out of fashion and nearly forgotten for a century or more after his death, Satie was not especially appreciated at his time, there are lots of other examples). In other fields too, e.g. consider Van Gogh.
Most of the works of obscure composers are known to fans of classical music (and musicologists), they are still occasionally played, but are still considered ho-hum.
I'd say good works tend to rise to the top, even if they're not popular at their time.
Where does the idea that music output is equally good in all ages comes from?
The fine art world, and the literary world know very well from historical experience of centuries that there are periods of huge masterpieces and lots of great creators, and periods of drought. Heck, the classical music world knows that too.
Why would this not hold in the pop music world?
Music is another activity that is historically influenced by period, trends, culture, etc, and all of these have their ups and downs.