I think it's simpler than all this is made out to be. For the last 200 years, we have been transitioning to instruments that can create chords by one player. Take a guitar for instance... its popularity as an instrument is that it is (relatively) cheap, plays chords, doesn't require a bow, and is enough to give a full melody and harmony alone. Orchestral instruments aren't designed that way, of course. Same thing with a piano. It's only getting more severe as electronic instruments can sample and play back chords of an entire orchestra through mapping and a keyboard.

Of course this isn't to say that orchestral music is unpopular because other instruments are more popular alone. You just can't "grow up" with an orchestra because it's not financially viable. Kids don't get in their garages to bang out string quintets... they grab guitars and keyboards. And once down that road, it's pop culture.

Orchestral won't ever make a comeback as long as it's more expensive to see than a rock concert, but because of its nature, it has to be. Musicians deserve to make money for their craft, but in the same way marching bands are festival and academically driven for these same reasons, the scale of ensembles matters for adoption.

Maybe just as important, orchestral music foregoes singing and lyrics, so each generation doesn't get a say. A guitar, a voice, a song, 3 chords, and poor lyrics will never replace orchestral as a genre, but it will always be the music of the people.

If orchestral wants to be relevant again, it's going to have to accept that 6-12 piece ensembles are going to be the way to make money. It will never replace 85 musicians in a hall, but acceptance isn't the name; it's the evolution towards relevance, toward profitability, towards embracing electronics, and towards the idea that orchestral as a genre is timeless - and must convey itself that way by interjecting into relevant popular music.

And before you say "wait wait that's not possible", that's exactly what JAZZ does.