is the playerbase beeing underestiamted? yes.
is vancian casting sitll dumb? also yes.
Well put, though I think it is fair to assume that potential BG3 audience is far wider then DnD fans. Personally, I never liked vancian casting and in BG2 for that reason Sorcerer was my prefered character, even if he was far less flexible then a Wizard. Personally, I thought Pillars of Eternity system worked really well.
There is one thing about vancian system which I fundamentally dislike - to use it properly you should know whom exactly you will be fighting. If there are particular spells I am expected to have, I would rather just have them, then have to go through a chore of going to sleep and rearrenging my spells. It might be fine if you manage one character, with with a party it becomes annoying.
Of course, the benefit of it, is that you can be given access to wide range of spells, while limiting what you can actually cast per rest. Meh.