I think people are overestimating the impact of behemoths. You can no longer take too many regions even with 40~50 nobles. The larger your realm gets, lesser the base tax rate. You will end up with a realm about 20 regions large at most with a lot of nobles. You won't get that much gold since your base tax rate would be 5~6% anyway.
The game already forces realms to be under a certain size in a way. I don't see the point of setting a limit on how large realms can grow.