I wouldn’t let the advice from that “well regarded coach” burst your bubble because his generalizations don’t hold true . . .
The shape of everyone’s PDC is different. How long you can hold any given power, including your FTP, is based on your muscle fiber type and other genetic factors. You can improve the curve “substantially” (shift to the right) with fatigue resistance training (e.g. FTP TTE from 30 to 70mins). However, as Tim Cusick (WKO4 product lead) shows in his video on the topics of fatigue resistance, optimized intervals and elsewhere, you can have 2 different athletes with the same FTPs, but very different shapes of their PDCs, and thus the %s the coach provided you would be very different.
Regarding 5WpKg, I would suspect that a fairly substantial majority of the folks at 4.5WpKg and above can do 50mins+ once they have done the training at their current FTP levels due to the amount and type of training it takes to get to where they are at. I suggest perusing this thread: