yeah but, even without rapping, Eric B/Dj Polo and them were most probably involved in choosing the direction, concepts, and sound of their albums. it's not the same as making an album as a solo rapper.
I can't speak on DJ Polo, but Eric B. didn't really contribute anything, at least in terms of the music. Rakim said in his interview on Crook's Corner last week that he produced 80% of his songs during his career. He made the beat to "Juice (Know the Ledge)" by himself. Eric B. was more of a businessman than an artist. That's why in most of the Eric B. & Rakim videos, you just see him with the

face in the background while Rakim raps.
LL was the first rapper to figure out what Michael Jackson and Madonna were doing. You have to constantly reinvent yourself, or else you become stale and get left behind. Each MJ and Madonna album introduced a new sound and LOOK that demanded attention. Consider the hat/glove of the Thriller era, vs the black leather jacket and zippers of the Bad era. Sometimes LL's moves didn't work...but remember even 14 Shots To The Dome, which was largely panned for the faux gangsta shyt, went gold.
With Slick Rick...I saw a somewhat recent GQ video he did, as well as the Drake video appearance. He has the same look. And now of course as a man in his 50s/60s, he looks utterly ridiculous. You can't stay the same in this game, or else you fall off and look stupid (no offense to Slick) A lot of black artists seemed to abandon the MJ model until somewhat recently. Beyonce, Rihanna, Drake (to a degree) and Kendrick have all adopted it in the last decade. Also...Weeknd. Consider the look and sound of his new album, vs the StarBoy era, vs the other stuff. And the way each era introduces a different hair style. That's pure Michael Jackson (and Madonna) shyt.
You made a great point. Hip hop has always valued image and style, especially back then when everything was moving at an accelerated pace. You either kept up with the constantly changing landscape, or you had to step aside for the new school of rappers. Run-D.M.C. learned this the hard way because in 1986, Raising Hell went triple platinum and they were the biggest stars in the world. Their next album came out two years later and sold half that. By 1988, they were already starting to get phased out as the kings. They tried adapting to the times with Back from Hell in 1990, but it was too late and hip hop was done with them. It took them completely rebranding themselves as legacy acts to get their momentum back. They stopped trying to be the kings and embraced being old.
I think it can be boiled down to something as simple as release frequency. KRS and LL came out every year or every other year. BDK, Rakim G Rap took longee breaks in the 90s. BDK came out with Prince of Darkness in like 91', his next album was like 4-5 years later. Rakim took even longer.
Kane wasn't taking that long to release albums. He dropped something every year from 1988-1991. Then he dropped in 1993 and 1994. He didn't take a long break until after the '94 album. His problem was like
@Piff Perkins said, he made the wrong moves and appealed to the ladies too much.
Rakim couldn't release anything because Eric B. wasn't signing off on his deal to do a solo album. It was Eric's idea to give the label a Rakim album, an Eric album, and then one last group album so they could get out of their contract. But Eric never gave the clearance to the label that he was done with his solo album, because they were only handing out the budgets one at a time. Because of that, Rakim couldn't go solo, couldn't do shows, couldn't do anything for months and Eric just disappeared.
It took a long time for Rakim to get a solo deal, but if all that didn't take place, he probably would have had multiple albums under his name by 1997 (which is when The 18th Letter came out).