I don't think the idea that: "All the good music has already been created,played,copied, completed... ," is valid. I don't think we've even scratched the surface.
I'll give three reasons why there is plenty of good music left to be created, and why it sometimes seems that all the good songs are taken:
Reason #1: The mathematics of it all.
Armed with only a little music theory, one can rule out vast swaths of potential songs on the potential, musical landscape. For a song to be good, I think it's save to assume that it must have some semblance of rhythm and melody. I'd say the vast majority of random combinations of notes can be ruled out.
But what's left is still huge. Enormous even. We haven't even come close to scratching the surface of what's left.
For example, maybe sometime in the future there will be a song, or maybe even a genera of music, that was originally inspired by some sort of combination between, say, today's Spanish Flamenco and Blue-Grass. It hasn't been explored yet, but the future wide open for such unexplored musical frontiers.
Reason #2: Every good song is inspired -- at least a little -- by other songs.
No good song comes out of the void. Every good songwriter and performer has his or her influences. A composer can positively add to the genera with his own little quirks and tweaks, but it invariably never comes from nothing before it. If a song sounded so different than anything already out there, no radio DJ (or orchestra or streaming service, etc.) would ever play it because it doesn't sound like anything that people listen to. Songs get played (get airtime, etc.) because they sound somewhat similar to other songs that people like at the given time.
As such, all new music (that has any chance of publicity) is going to sound -- at least a little -- like something that came before it. But that doesn't necessarily mean that it's not a new and good song. A new song can still be good, even if the artist had clear influences.
Reason #3: At any given point in history, nearly all music produced at that time was god-awful crap.
Yes, I said it.
Your 20-year-old musician friend might say, "Music that was produced in the 1970s, '80s, and '90s was alright. I know because I often listen to an 'oldies' radio station that plays only music from the '70s, '80s, and '90s. But all the stuff today's bands are churning out now --and I listen to some of that too on the local Billboard Top 40 station -- is all derivative crap."
What your young friend might be neglecting is that the "oldies" radio station is only playing the good songs that came out of those decades -- those songs that stood the test of time. (And just to be clear, being #1 on a Billboard Top 40 chart is a weak indication on whether that song will stand the test of time. They are very often not the same.)
So when the young musician says, "the new music coming out today is derivative crap." Yeah. He's right. Nearly all of it is derivative crap. He's correct there. But that's nothing new. It's always been that way. Hundreds of years or more, it's been that way.
At any specific time during the '70s, '80s', and '90s, for every one good song by AC/DC, or the Police, or REM, we had to listen through dozens of crap songs of the likes of somewhat-country-yet-somewhat-rock-and-roll-duet, through a seemingly endless stream of hair bands. Oh, god. The hair bands. Do you remember the hair bands? Of course you don't. Nobody remembers the hair bands because we have correctly, and thankfully, eliminated the god-awful hair band songs from our collective memory for the greater societal good. It was god-awful. God. Awful. Jaysus.
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But hope is not lost. For every few dozen god awful crap songs that come out today, there's a gem hidden in there somewhere.
Rest assured that in 2050 there's going to be a 20-year-old who says, "Back in the 2000s, '10s, and '20s, the music was alright. But the music they're churning out today..."
