His response is a little too gas lighty for my taste.
He seems to be trying to manipulate you into doing what he wants
If I trusted the vision , like Prince or Bowie level results, I might tolerate that kind of attitude
But you clearly do not
trust his vision
I don't have the heart to tell him this
You stated this twice in the OP.
What you need is not
heart but the
courage to be honest.
You (maybe) fear speaking the truth because you fear you have something to lose.
Maybe you fear loosing his friendship
Maybe you fear loosing a musical outlet
Maybe you fear being
the jerk who broke up the band
And rumors about your unreliability
all the bad things could happen
so steel yourself for the worst ( have a plan )
and tell him the truth ( tactfully, kindly )
A stable friendship can withstand such things
Ya'll sound young to me. I'm old, secure, and have less Fs to give, so I'd tell him his vision isn't all that and let the chips fall.
( or, if I felt his vison was "half" of all that, I'd offer to help improve the material )
I'll also just point out that a bass player who does covers and can sing (especially back up ) is an easy hire for any paying band.