It's definitely that to an extent, but also the low quality songs were less likely to get released in the first place back then because the barrier to entry in the music industry was a lot higher. There were just as many people making low quality songs but fewer had the ability to record and distribute them.

I can think of hundreds of great tracks from that period. I've managed to discover perhaps an equal number from after, but I had to dig really hard to find them. The "popular" stuff these days is chrome-plated garbage.

I remember reading somewhere that people prefer music they first heard around ages 13 - 21.

Just curious, what age were you when you first heard those tracks?

That's true for me. Unfortunately, I'm an outlier in the sense that I heard virtually zero music before 13 (as if I were living on an Amish farm or something).

Moreover 1960s to 1990s is a larger time frame than from 2000 to 2019

I'd put the golden age at 1969-1984 or so, but that's just me.

For a few simple examples, look up King Uszniewicz & The Usniewicztones, Legendary Stardust Cowboy, or The Shaggs.