There are actually some well-documented factors that play into this issue of today's music being different and/or worse. Some of it is perception, some of it is exposure, and some of it apparently is real.
When it comes to Top 40/Hot 100 music - the most popular of pop music - scholarly analyses have been conducted, and both the lyrical and musical components have indeed gotten simpler and less varied in the past 30 years or so. So that part is real. But on the other hand, this dumbing-down trend has not been observed more broadly in the musical landscape - it appears to be mainly in the realm of "hit" music, which has always contained a lot of pap. I mean, look at the Top 40/Billboard charts in the 1970s: tons of amazing music was getting made and selling like hotcakes, and yet the charts tended to be filled with a lot of very lightweight pop.
The industry has also changed, and when it comes to pop/rock music in particular, AOR (album-oriented rock) is not the dominant format that it was from the 1970s through the mid-1990s. A lot of that is about the rise of digital music, first with individually purchasable songs and then with streaming. Those are both media that strongly favor singles and songs over albums, and encourage Endless Shuffle playback. These media have also ushered in the decline of traditional radio overall, and with it the decline in deejays and critics as tastemakers. It's part of the larger fracturing of all mass-media/entertainment audiences. So there's less support for and cultural/economic centrality to the album format, which also means shorter songs and less varied types of songs. And there's less exposure to album-based music, so the varied, interesting albums that are out there are generally not getting heard by as many people and are not occupying as central of a place in our collective attention.
The above phenomena have also been exacerbated by the reduction in the number of people who are writing and producing the lion's share of the biggest hits. In a way it's kind of like a return to the pre-1970s era, when singles were the big thing and most artists performed songs written and produced by others.
The rise of hip-hop and its displacement of rock as the most popular musical genre has had huge ripple effects. The barrier to entry for hip-hop is lower than for traditional, band-based rock music (which is why hip-hop began in the first place), and it has always been based on samples and rhythm more than on solos, instrumental breaks and changes, complex compositions, and so on (the best trip-hop is a partial exception to this). The rise of inexpensive computer audio workstations has only intensified this trend.
On the other hand, the production and creation of music has never been more widespread than it is now. Tons of fascinating stuff is getting created - it's just coexisting with an ocean of simple, homogenous product.
Oh, and mandolin > banjo. Just sayin'.
