Thing is, if a player cannot handle fullback for physical reasons, then that is the last kind of player that you want to pin your chief playmaking role on in the halves.
I think when you are talking about the marquee player of a club, and you are looking to hide or protect them, then you have probably made the wrong salary cap calculus at the time of signing that player for a three year deal.
All things being equal (taking the benefit of hindsight out of this) I would expect the Warriors banked on Metcalf being robust physically, and in line to make a full recovery at time of signing - and well capable of playing anywhere outside of hooker in that spine.
For now your concerns make a lot of sense, but once Metcalf is shown to be fully fit, then I reject the principle of hiding players because of what might or might not happen, Metcalf plays a running game anyway and he plays up in the line, hamstrings and knees don't know the difference.
In fact fullback is a safer position to play in many respects in terms of accrued contact trauma.
Talk of Metcalf not being physically built for fullback is no different to the talk of Taine Tuaupiki being too small for the job (It is like no one has ever seen the movie Rudy).