I don't think that's the point of the OP. I mean, my interpretation is that the thread is about the period of time when the best groups and artist held strong popularity. At first I pretty much agreed with the OP, but after a bit further consideration I do think that the range of music considered in the OP is too limited and thus creates a flaw with the idea that the best popular music doesn't extend into the 90s. I'm trying to look at this from a perspective of the best popular music not just being music that I liked, because certainly much of the popular music in the 90s I did not like.
I think we can break it down into a core period and then also into a longer extended period. The core period I believe could be from 1971 - 1982, made up principally of the following artists: Michael Jackson, Led Zeppelin, Pink Floyd, The Eagles, Meat Loaf, Fleetwood Mac, AC/DC, and Simon & Garfunkel. Prior to that period it was primarily The Beatles, from 1967 and onward well after the band split up.
But there's a much longer stretch of time that extends well beyond the 80s, all through the 90s and even into the early 2000s.
Looking from 1967 - 2002, you have:
The Beatles, Led Zeppelin, Simon & Garfunkel, Pink Floyd, The Eagles, Meat Loaf, Fleetwood Mac, Michael Jackson, AC/DC, Queen, Madonna, Phil Collins, Whitney Houston, Bon Jovi, U2, Mariah Carey, Celine Dion, The Backstreet Boys, Eminem, and Britney Spears.
Now many of those I just do not personally like at all, but if we allow huge "popularity" to be the determiner of what music is the best, then certainly all of those artists were hugely popular at points during that span of time. At least one of those artists produced a hugely popular album during that stretch of time in every year accept: 1974, 1978, 1983, 1988, 1992, 1998, and 2001. That's 29 years out of a 36-year period in which hugely popular musicians were presenting new music for public consumption.