True heads know that Rakim's best work was on his first three albums with Eric B. And that was basically a little over 30 tracks about The God, and how he viewed wack MC's, the good life, the streets, life in general and current events. This is the basis of content for most of the GOAT's. How often they choose to engage will obviously vary, but you can't discredit one great because he doesn't follow in the same creative process as another.
KRS isn't Scarface. Just like Kane isn't Rakim. Vice versa. Nas won't be B.I.G., the same way Cube won't be Pac. What makes a great MC, is being great at what you do, not building the same kind of catalogue as another artist. Today, people want to compare everything track for track. But you'd never see anyone saying PE wasn't as great as De La Soul because they didn't have a track like "Saturday's". The culture wasn't built on one lane. So there's room for EPMD to be EPMD, and Outkast to be Outkast without needing to have a track like ATCQ's "Excursions". LL never had to be anybody but LL. Same could be said for G Rap or Slick Rick. What makes them great is individuality, not how they compare to each other.