It's silly to claim the quality of music is decreasing. People have been saying that for thousands of years -- even Plato said it. I hear it every year, and it's never any more true from one year to the next.
The fact is, there is a ton of good music being made. In all likelihood, I'll still be listening to the latest Grizzly Bear album in 25 years -- probably looking back on it with the same reverence and fond nostalgia with which people view Zeppelin, Clash or Talking Heads albums (all of which I love. You say you stopped finding "good stuff" in the early 90s, but Pavement's first album didn't even come out until 1992, and I'm sure I couldn't live without it (well, maybe that's a little hyperbole) or the one after it or many other 90s albums.
I think I read somewhere that as we get older, our brains don't form the same emotional connections to music as they do when we're teenagers or young adults. I would imagine that has a lot to do with why my dad asks me to download Joni Mitchell songs or why my Grandma still listens to music from the 40s and 50s. And probably with why you can't find good music anymore.
Doubt is a pain too lonely to know that faith is his twin brother. - Kahlil Gibran