The reason why I don't use AI or even code completion

this is true as far as it goes, if you don't use it you do in fact lose it

but i've convinced very very good engineers to switch from vim to a "real" idea and their productivity went through the roof

do they have a worse memory of the stdlib? absolutely

are they spending those brain cycles doing something more productive now? also absolutely

where is the line? idk but for me personally I haven't found it yet.