Yes, the DNC can change the rules whenever they want. But when your party is supposed to be putting a priority on Protecting Democracy, it’s a bad look to then say “Yeah, we held a bunch of uncontested elections which we are now ignoring the result of”.
Biden clearly does not have dementia. He has some other serious problems, but his mind is there. Once you decode his halting delivery, the ideas behind his positions are sound. You can tell his mind is still sharp, it’s his body that is betraying him.