All decades have shit mainstream music but also a lot of great obscure music that nobody ever heard of. I’m sure there’s a lot of good music today. I just haven’t found it yet.
I think the 80s was when mainstream music started to suck, when progressive rock groups (or rock groups in general) started writing more radio-friendly music (even King Crimson did this to some extent). Then (some) “musicians” started using autotune, drum machines, and everything was done on a computer (no longer do they play instruments). They lost any sense of creativity or musicianship and are simply getting paid for doing nothing. How the fuck did artists like Justin Bieber and Taylor Swift ever become famous?