The BBC regularly repeats old episodes of Top of the Pops, a music chart programme that was broadcast between 1964 and 2006. It's quite close to a random sample of weekly Top 40 listings - about half the episodes in the archive are unsuitable for repeat broadcast, because they were presented by notorious sex offenders.

There was a lot of crap in the charts, but I think the overall level of quality was higher. I think the reason for that higher quality was essentially negative - the routes to an audience were much narrower, so artists were practically required to get into the charts to sustain a professional career. A far broader range of artists were playing the major label game, because they didn't have many alternatives.

"Pop" is a much narrower term in 2018 than it was in 1978, because musicians don't need chart hits or radio airplay. Digital recording and distribution have created a much more fragmented music market, because artists can connect directly with audiences rather than relying on a very narrow funnel of publicity and distribution. Good music is somewhat more difficult to find today, but I think there's more of it and more definitions of "good".

https://en.wikipedia.org/wiki/Top_of_the_Pops

(proxy required outside the UK):

https://www.bbc.co.uk/iplayer/episodes/b00704hg