It's only my opinion ... but no, I don't think technology has decreased average quality. I think that other forces have sort of driven the use of technology that way.
It's like when I looked at my daughter's Hillary Duff CD and noticed every single song on the thing had been written by a committee. Technology didn't drive that -- it just facilitated what the "make Hillary Duff a singer" project wanted to achieve. But I think it is forces like this that drive the decrease in quality.
I am quite sure that due to the magic of Internet and free soundcloud and self-publishing music through Amazon etc that there is a far greater quantity of poorly produced material available than has ever been the case. It's sort of like how Facebook has made it so people who couldn't pass 9th grade English get to write where the world could theoretically see it.
So taken as a whole with "all available produced material" being averaged ... I am quite sure that the democratization has indeed lowered that average -- perhaps substantially.
So here I am ... contradicting myself and saying technology has enabled decreased quality? Not exactly -- here is why.
The Hillary Duff cd had media promotion, advertising and serious money, names, name recognition and the credibility of a major publisher behind it. People actually HEARD it.
But 20% of the songs on Spotify ... have not even been listened to even ONCE -- so even their creators haven't listened to them via that portal. If Joe makes his latest creation and uploads it to his soundcloud account -- the only people who will ever hear it other than a random will be Joe and his buddies. That music is technically available and if it is horrible it lowers the quality, but in practice it is statistically insignificant in terms of what we hear on the radio.
The quality of what we hear on the radio has gone down, but I don't think the technology is what drives that. What we hear on the radio generally has money and backing behind it, and the practically infinite resources of media giants. If the quality is low, it is not because Joe fired up a daw and played his guitar into it through a radio shack mic. Nobody hears Joe.
If you think about it, there have been a large number of "uncoverings" where it has been discovered that major hits from the 50's and 60's had singers, guitarists and drummers other than what was advertised. They just had some great singer do the singing but sold it under cover of someone they considered marketable.
The fact that they do that today with software just changes the means, but not the motive.