As usual, not that I try to but it usually just shakes out this way, I'm gonna have the opinion that induces the most rage.
So before we begin
Okay let's do this.
Firstly, I'm not actually certain that what you describe โ composers and musicians not showing up to concerts โ really matters. Actually, I would say it's often better if they don't.
I, for one, don't actually make music for musicians to listen to; I just want normal people to like it and listen to it. But I find getting musicians to listen to new music is no problem. On these forums or whatever, I can usually expect to get a few dozen plays or something. On YouTube, some of my tracks wound up getting into the recommended feeds of normies and skyrocketed to tens of thousands of plays and hundreds of likes.
I am always glad when musicians, especially ones much better than I am support what I do and like it, but it means a lot more to me when the average person, who just got off their boring, soul-crushing 9-5, stumbled upon my music, thought it was freaking awesome, and shared it with their friends because it brightened their day that much. Maybe, it even inspired them to get into composing themselves and that's even better. Then, a beginner reaches out to me via forums, email, IRL, whatever and is asking me for advice on their own stuff and how to do something. That's also humbling and good and it's actually gotten to be a bit too much this past year (which is why I'm making some online courses and prob gonna offer some skype lessons soon) too keep up with.
All of that, in my opinion, is far better than musicians showing up.
I played in rock bands and let me tell you something: The brutal reality of 99% of live music today falls under two categories
โข It totally sucks. Standards are very low. I can't tell you how many years it took before I could get to jam with a band that could actually play IN TIME with each other and would show up. I played alongside a lot of bands that just friggin' sucked, dude. They played way too loudly, too much distortion, singer couldn't sing, out of time, etc. Then, after the show they'd come up to us and be like "Hey, you guys were awesome, man!" and we'd say "Thank you" and then were always like waiting for us to say they were great too and looked disappointed when it didn't happen.
It's like, well bro...I'm not gonna lie to your face and say you were amazing when you clearly bungled the song, don't know how to write a coherent piece, and couldn't even play in time with each other. If someone came up to us and said "Hey, you guys kinda sucked tonight", we were willing to be like "Yeah, it wasn't as good as it should have been".
Most musicians don't have that kind of self-reflection, so what happened at virtually all of these shows, is that the "audience" was 100% "musicians". Just guys in bands that suck as bad as 9/10 of the other bands. It's a total echo chamber and "trophy-for-everyone" fest.
People don't come because the bands aren't worth seeing.
โข Like with art galleries, there are many talented people out there.
They don't get promoted though. If you have some sort of social justice message to sell, you can just literally tape a banana to a wall and they'll promote the crap out of it for purely political reasons.
The guy who can actually sculpt, compose, paint, etc? Nope. Not interested.
The industry is full of gatekeepers/politicians.
This then harms the actually-good composers when they do get promoted, because most people just assume it's more modern-art garbage.
Now I will say, I'm not entirely sure what it's like in local orchestral or chamber music "scenes" as per point 1, but I know it's true for point 2.
The local orchestras near me almost never play anything by new composers and the few times they seemingly have, it was pretty much not even advertised so I never would've known about it anyway.
Last point
โข You have a niche category of music
Most people today, including musicians, outside of a film/game context, do not have any interest in orchestral music.
Even I can honestly say I don't feel compelled to go watch a string quartet + winds duo.
People want to have fun at concerts as much as they do hear the music. People want to dance, they want lights, they stage antics and performance, they want the social aspects of it as well as the music. Sitting silently in a cathedral or hall to listen to a cellist saw away might sound nice and have a nice atmosphere to it, but it's just not something most people are going to make an afternoon/evening out of, or certainly pay for, when there are (at least pre-2020) so many other more-fun things they can be doing.
Historically, the orchestra was generally the music of rich people and to a lesser extent, churches. For the peasants, the unwashed masses, the normal people โ the fiddler on the street or at your spring festival and drums is what they know as music and they'd have much rather been doing that than going to sit quietly in church and listen to a choir sing ANY piece old or new.
In other words? Musicians aren't showing up to local concerts? Good. &*#@ 'em. If it's mostly musicians showing up, it means that the music is pretty bad. If you have like 40 normal people who just want to listen to music showing up to an orchestral/chamber music concert in 2021? You're doing very well, I'd say.