i dont agree with that. I believe in a Kellen Moore type offensive that his arm talent can improve and eventually equal his leg talent. It is true that over time the player loses his legs first. But im talking about short term, not 12 years from now. Look at Bryce Young at Carolina, its taken him 3 years to finally develop, i see Justin Fields the same way. But justin Fields can be better because hes got better scrambling ability.
Bryce Young has only been in the league 2 years and he showed signs of development in his second year. Fields has played four years and still hasn't shown growth. He got benched for a guy whose old team paid $40M to trade him lol
The graveyard of the league is littered with guys with leg and arm talent. The trait they lack is between the ears and that's what Fields problem is.