More good points here. I think it is objective that at certain times music is dumbed (and as a matter of fact I totally subscribe your comments about late '90s music). The reasons why it is so are most probably multi-factorial and not always easy to devise and describe. Politics are also involved in my opinion, at least to some degree or transversally.
In the case of the '50s in Italy, it was clearly politically driven. A new democratic order had to be estabilished after the fascist disaster, and the most cheesy things that had to do with family, marriage, children, religion and "well wishing" in general were emphasised if not directly sponsorised. All of this was clearly reflected in music, which doesn't mean that there were no longer cool jazz clubs downstairs, but they were niche stuff, and the music played there very rarely went on records.
In the '70s, I believe it is quite undisputable that the entire western world was pervaded by a political desire for change and progress. In this scenario, culture was considered an important pathway to freedom and self-fulfilment, and pretending to be intellectuals (or supposedly so) was even fashionable. Families even from the lower classes spent considerable sums to buy a Hi-Fi system, as listening to quality music was considered an important part in family entertainment as well as in children education. Progressive Rock in particular found a fertile society in which to develop and be appreciated.
The '80s bore witness to the tremendous disappointment that followed the illusions of the '70s, but the New Wave nevertheless managed to intercept the changes in a society increasingly devoted to selfishness, as well as the buzzing city life so fashionable in those years, during which it was widely believed that financial growth could be limitless.
It is hard for me to understand how it could happen at all that in the '90s such a vast society so used to listening to good music had ended up taking their Hi-Fi systems to the basement, the antechamber to the dump. In a way this was also "politically driven", in the negative sense that nobody moved a finger to at least culturally guide a society destroyed by economic reflux and displaced by the so-called "globalisation". In this scenery, you are most probably right: other media/interests eroded the time that was previously devoted to music listening, like the early chats on internet (that were literally a time well), and later the social media and the digital pay-TV, that ended up disintegrating the only socialising aspect of analog television: that is to watch the same TV show that also most classmates or business colleagues watched, and then comment it at school or at work.