Spoilers
My favourite sorcerer that has incredible charisma is this guy from Star Wars, Emperor Sheev Palpatine.
I will be honest with you. Were i in a galaxy far, far away i would admit this guy was an incredible leader. He doesn't just run for high-school president, he runs entire galaxies and prepares them for war against other galaxies. In fact, no matter how powerful or smart (or even which side) people were on they ALWAYS, invariably, worked for this Sheev dude. That's just uncanny.
Would i want to have him date my daughter? No, most definitely not. Would i want him to rent a room if i owned a large hotel? No. How about just having him live in the neighbourhood? No - and in truth, i would not want to be living on the same planet as this Sidious. That would just be a terrible situation.
Honestly, i don't even want him walking my dog. I especially don't want him walking my dog.
I like your argument, i really do. But as far as that whole sorcery-warlockish vibe, i think our pal Palpatine has that covered. I also feel that he has charisma well out past the 30 range once you factor in his mind-control stuff that seems to work on ANYONE. Heck, it even messes up Yoda to the point where the wee green Jedi 'master' confronts him... and then does not defend himself. 'Your arrogance blinds you! I am going to shoot soon! I mean it! Defend yourself. Please? Okay. Well, then. If anyone asks... warned you, okay??'
Charisma and then even more charisma, out the yin yang and then some.
This is also how i figure other charismatic yet revolting creatures work. Hags can be profoundly charismatic, somehow managing to avoid entire towns of pitchforks and torches yet never getting so much as a text message. Does anyone ever dated a harpy? Lots of charisma!
It seems that D&D is very good at sending a message that one's 'charisma' may not be for everyone. This may also apply to 'wisdom' (you can have 30 and still be Chaotic Evil - so much for enlightenment?) and 'intelligence' (at 20+ intelligence, great dragons never do much with it). The three pillars of mental prowess never translate into motivation, wit, cleverness, charm nor persistence. That's why this is a game!