Of course 90% of yesterdays music was utter crap, and ten years ago, and twenty, and fifty...
Hell, I'd watger that Sturgeons Law (at any given time, at least 90% of anything will be utter crap), applies to popular music more than any other field of endeavour. For every Beethoven, there are about 999 Gilbert O'Sullivans.
The fact is, popular music (by which I include all genres sold into the mass market not just "pop") is calibrated and calculated to be sold to four groups, in descending order of market size and power:
1. Suburban girls from age 12-24
2. Suburban boys from age 12-24
3. “Urban youth”
4. Gay men under 40
The vast majority of “pop” music, is calculated to appeal to girls in the 12-16 age range, and to gay men, because they buy more of it than anyone else (combined with the rest of the 12-24 females, they make up nearly 100% of the pop purchasing market). That is what determines the majority of what is played on MTV and top 40 radio.
The majority of so called rock, is calculated to appeal to 14-18 year old boys, and 16-24 year old girls, because they buy most of it (again, combined they are nearly 100% of the buying market). This is what gets played on "rock" radio, and a small portion of what MTV plays
The majority of hip hop, rap etc.. is calculated to appeal to suburban white boys, 14 to 18, and “urban youth” 16-24, because they buy most of it; and this is what is played on "urban" radio, and the rest of what MTV plays.
Those demographics cover more than 80% of the new music buying market, and thus control what record companies produce, radio stations promote, MTV plays videos from etc…
Oh and it should be noted, those demographic numbers haven't significantly changed since the 1930s… except to get worse (i.e. they are a greater and greater percentage of the market, and the "gay men" demographic became a significant market force around the time disco became popular), as disposable incomes have risen, and purchasing power has transferred from middle aged men, to young women, and children.
The music which those demographics have found appealing has changed, but the groups haven't; and thus their music has become the popular music of the time in all cases.
It should also be noted, those demographic groups aren’t exactly known, on the whole, for having good taste, refinement, or an attention span greater than 3 minutes and 30 seconds.
Mariah Carey has had more number one hits than anyone except The Beatles, which really should tell you something.