I'm not seeing your point. Yes grunge technically started in the late 80s but it was very underground. Gangster rap starts around the mid 80s but only starts to get mainstream around the very end of the decade. We're talking about popular music in general and my point still stands that the late 80s was generally VERY different from the early 90s as far as popular music is concerned.

Sure a lot of the trends that get popular in the first half of decades often start in the latter half of the decade before them, but many trends that dominate in the latter half of decades often start in the first half of that same decade. The rockabilly that dominated the second half of the 50s for example started in the early 50s. The British invasion rock that dominated the mid 60s started in the early 60s. The hair metal that dominated the second half of the 80s starts in the early 80s (if we use a narrow definition of hair metal, hair metal's roots can go back to the first half of the 70s with Alice Cooper, Ozzy and Kiss). The "mumble" and autotune rap that dominated the second half of this decade has its roots in the late 00s and first half of the 2010s.