I don't think we have any insight into Goff being a bust. He hasn't even played one snap. Jared certainly didn't play lights out in the PS ... but he also never got to truly play with the 1s. He played with a few of the 1s but never the entire starting offense. Fisher never gave him a real shot at the starting spot IMO.
Could Goff be a bust? Possible.
Could Goff become a Franchise QB? Possible.
IMO, if Fisher remains as coach then Goff has a much higher probability of being a bust.
If we hire a offensive head coach that knows how to develop a QB then Goff has a real shot to become very good to great.
