One reason for this pattern is classism: the novelties of the upper classes are held up as glamorous and even virtuous, and that goes away when something is mass-marketed and becomes popular among the lower classes. Eventually, one might even hear words like "disgusting" and "poison" used to describe something that was once considered perfectly wholesome.
Both media began with precious little content, most of it relatively high in value for the times and circumstances. (Even if it could be argued some content was low quality or utility, the simple access to a much wider array of thought was valuable.)
But once they matured, of course they filled up with trash. Just like every medium.
Given the quality of most books these days, someone “reading a lot” doesn’t mean much anymore, for instance.
1990: He is so dumb, he is watching TV all day
2000: He is so bright, he spends a lot of time on the Internet
2020: He is so dumb, spending all day on the Internet.