How did any of those brands diversified and then novaed? They all were damaged by too many movies, not ancillary products like makeup.
Did GI Joe have a plethora of movies in the 1980s that I missed? Or Transformers?
No, these IPs - which ruled kids marketing in the early 1980s had their life choked out of them by the IP owners pushing for monetizing the IP too aggressively. They knew that they would be better off squeezing the IP for every penny, letting it die off, and then squeezing the next IP the same way. That squeezing took many forms.
There was no internet in those days, but there were malls, Target, Sears, etc... You would find GI Joe on everything. You'd also find them constantly overturning the roster of GI Joe in the shows and comics to highlight new characters and vehicles so that they could sell new toys. However, kids didn't want Snow Job, Muscrat and Banzai ... they wanted Snake Eyes, Scarlett and Hawk from the earliest generations. The bait and switch resulted in a loss of interest in the show and the products.
The kids didn't want the GI Joe Ice Cream, GI Joe Shirts and GI Joe Pencil Cases. By the late 80s this stuff was being offloaded in clearance stores all over the place - and it was about that time that the animated show really died. They tried to bring it back in the early 90s ... but it had no identity and lacked the capability to draw in kids.
The same types of things happened with Transformers, TMNT, Care Bears, He-Man, Rainbow Bright, etc...
To keep an IP going strong you need to be true to your core identity, you need to maintain quality, and you need to focus on long term growth over short term profiteering. If you were to, say, flood the world with your IP so fast that quality took a dive while simultaneously turning away from your most Marvelous characters in favor of 2nd tier characters .... while simultaneously flooding the world with your merchandise all the time to the point we get numb to the faces of the characters ... It isn't like that would end the dominance of an IP in a SNAP.