I think a big part of the deterioration of musical quality is connected to three major factors:

1. Recorded music has meant less need for such large numbers of musicians, as a result the pool of potential talent is lower. Much less live music is performed as a result, as a result the pool of musicians with great live musical abilities has gone down.

2. Multi-track and then computer recording has made it possible to make much more 'effects' based music, which wows the ears sonically, but requires less musicianship to back it up. The large producers of pop music has largely clung to using these techniques to give their music a unique 'modern' sound, largely fueled by the latest developments in technology.

3. The ever progressing corporatization of the music industry.