Judo relies on short energy outbursts and a very specific form of cardio.
MMA is much more demanding in that aspect. He would not be able to obtain his 140kg frame and no one knows if he may adapt especially to the stand up requirements.
As someone who trained some years in Judo I have to say that wrestling sharpens you much more for mma. You have the throws, leg attacks, ground work, you have also excellent clinch work, top control and wrestling is much more cardio heavy. After all wrestling offers a more complete package to go into mma ( I say this as a mainly judo practitioner in my youth).
Riner is very good but to put things a little bit into perspective the playing field at HW in Judo is much more thin and weak compared to wrestling. Because of that there really are no comparable physical challengers for him. Even today Japan is very strong in Judo and they just are smaller men in general.
Nastula showed that its possible to cross over from Judo and have success but you need the ability to take a punch and develop stand up.
Everyone thinks he can do that but only few exceed.
Riner can put his size advantage to excellent results in Judo. That would be much more difficult in MMA and as someone who never got used to being hit I would not see him as more as a promising prospect who would need too much time to adapt to MMA to make it to the top.
Regarding Karelin that would be a whole other topic. Were Riner is very strong Karelin was just out of this world. He trained harder than any other ahtlete and played with world class wrestlers like children. Karelin was 10x better than someone like Brock Lesnar and look how far it got Lesnar.
I dont even see Werdum catching him in a sub because of how otherworldly strong AND technical (dont forget that) Karelin is in wrestling.
The trainer of Barnett (dont know his name) once showed his kind of body submissions by applying pressure on the leg or belly. Someone like Karelin would easily tap out everybody with that or just simple submissions powered through. Picture a Kimura or neck crank from Karelin. Frightening.
Look Bob Sapp vs. Nogueira up. Sapp only had strenght and no techniqe at all. He nearly killed Nog in the ring before gassing. Karelin wouldnt gass and imo after 2years training catch the HW belt and held it as long as he would like.