IMC great wyrm red dragons, titans, and the other CR 20+ monsters are all named, rare and important beings. I never took it as a requirement that all unnamed creatures are common. No cities of Titans IMC. This could go for Balors as well. In 1st edition, there were only six known Type VI demons.
I think a strong opinion about the demon lord CR issue requires an opinion about abyssal demographics. A CR 20 creature is completely capable of dominating CR 14 demons and weaker, of which there are plenty in the SRD at least. With groups of CR 14 creatures thus dominated, the loyalty of CR 17 and 20 demons is more easily assured.
The idea that demon lords would have to battle more than one demon single-handedly IMO is not understanding politics - even chaotic evil politics. Granted, there's no sociological study of Chaotic Evil that I know of. However, there are a number of points I think that contest this picture of Yeenoghu having to beat all of his underlings simultaneously in order to justify his rulership.
Nobody wants to be the first balor to step out of line. The other balors will kill that balor and say "He was going to rebel against you m'lord, can we have his stuff?" Anyone who says they're going to join your rebellion is possibly lying, and the price of failure is eternal torture, so who would take the chance? No complicated creed required, this is just simple self-preservation in a dictatorship.
Any two balors wanting to rebel know that they will eventually have to kill the other in order to be the sole ruler (I can't imagine co-regents in the Abyss). This means that the two will probably be fighting each other before the rebellion is able to get underway.
Conceivably, there are many weaker demons who have important posts serving Yeenoghu. Those creatures would probably all be lunch and replaced were the "balor rebellion" to succeed. It's in their interest to spy on the balors and each other to make sure no one is going to mess up a good thing. (Again, I'm not sure Balors would even serve a demon lord)
Granted, you could take an extremist view of what Chaotic means in the Abyss and argue something like that demons would just go crazy every once in a while, heedless of the possibility of eternal torture, and just try to kill Yeenoghu as a mob for the heck of it. While that's possible, I doubt that any of the persons objecting to the CR 20 demon lord has given his CR 30+ demon lord so few followers that the same possibility doesn't exist in that scenario as well.
IMO at some point, any demon lord of less than CR 100 is going to have to competently manage politics if he has anything but a trivial set of followers.