I think the main argument usually is time savings. Personally I just always do E-Mail and password auth, yea its old and not the shiny new thing, but it doesn't require me to integrate 200 different ways of doing auth.
We should be able to demand users remembering their passwords, I dont like to cater towards users who simply dont want to put in the work to use my product.
Will I lose potential users over this? Yes. Does it feel bad knowing I am in control and wont have to offload to 3rd party vendors? Hell no.
Same here, Just email + password, no google dependency initially. If more users ask we will think of it. but again you don't need a cloud vendor for all this.