The entire framing of the proposition is wrong. The problem is not modernity or some downward trend in the overall technical and theoretical complexity of music, but rather what is presented through the major channels as being mainstream.
We live in the goldenist of golden times of music. You just have to search around for it.
Most of the criticisms of modern music could also be applied to "the good stuff" of years gone by. The most common popular music has always been equal tempered, diatonic and rhythmically based on powers of 2.
I think there's a rational case to be made that in fact the "millenial age" modern music is, in large part, nothing of note. And by that I mean where are the legends and icons? Of course there are good singers. There are always good singers all over the world. And with youtube we're starting to see them. But there's mostly a lot of people copying others' music all over youtube. Where are the ones who are not only skilled and talented, but also produce incredible, innovative stuff that will last for decades?
For example, Led Zeppelin. Bonham was arguably one of the greatest rock drummers of all time. Without doubt. And he was together with Page, one of the greatest rock guitarists, without doubt. And Robert Plant arguably one of the greatest rock signers. And the stuff they produced was like nothing before. incredible skills, incredible talents, and they produced stuff that was/is amazing.
Where's that now? All we get is some catchy tunes by folks who won't be remembered in 2 years. Where are the orchestras? The incredibly talented groups of folks who have been honing their skills for decades, and get together to produce something special, live and in person. Where are the musicians with actual talent? Good singers are a dime a dozen. And good singers copying others' songs are a dime a dozen. Where are the standouts?
I joke a lot about Billie Eilish, but for a good reason. She's a perfect example of the nonsensical attitude of "oh, it's a catchy tune, so she's awesome". Even though she can't even sing, she just freakin' mumbles. The bar has dropped so low that it's freakin' embarassing. I thought it was low with rap music. It just keeps getting lower. Where's the quality, the innovation?
Of course there's been crap throughout the ages. Hell, even millions around the globe fawning over embarassingly horrible crap like "Yellow Submarine". But there are also and unending list of examples of real talent and quality and innovation in past years. And not just the ridiculous Frank Zappa "innovation", but stuff that really changed things in a good way.
IMO, what's missing nowadays is quality, real skill and talent, and producing stuff that's meaningful and innovative. It's all just a catchy tune inside an entertaining video. And just like the youtube users who can't handle anything over 4 minutes without losing interest, everything in the music industry is now produced down to a few minutes. Gone are the long musical stories, the improvisations. Now it's all just bite-sized BS.
Would Stairway to Heaven even exist today? Hell, can you even find a Broadway-type show that isn't just 10,000th repeat of a show that started in the 70's or 80's? Do orchestral concerts even exist anymore? Yeah, there's the occasional catchy tune on the radio by some new, one-hit wonder. But that's not my definition of really good music.