I see this said a lot and as someone who lived through the 80's and 90's I just have to ask: If that was true, why did people in the 80's or 90's not think music mostly sucked, but people tend to think that now about current music?
These days any asshole can put their shitty music on soundcloud and buy a spambotnet to post it a thousand times a day. Back then only assholes with the right connections and money got their shitty music on the radio.
I can guarantee you there were people in the 80's and 90's who thought music was better in the 50's and 60's and so on and so forth all the way back through time since Grog hit two sticks together rhythmically
People who think this about current music simply aren't hearing/listening to a lot of current music. There's great stuff out there being created all the time but you'd never come across it in 'mainstream' places. Take a genre I really like (I realise not everyone does), blues guitar/vocals. 3 brilliant current artists:
Grace Bowers (will be 18 in July)
Christone "Kingfish" Ingram (currently 25 years old)
Muireann Bradley (also currently 17 years old)
Obviously with those ages, these aren't golden agers coating on past glories. To take someone totally different, Ren isn't 'commercial', even if some of the people he's worked with, e.g. Chinchilla, are. I don't expect to see any of these artists become 'mainstream' like e.g. Ed Sheeran or Taylor Swift.