Someone mentioned Baker and Darnold earlier - both these guys were tossed aside by their drafting teams, had to take on backup roles but then found some coaching/maturity to turn the corner.
QB's are playing into their late 30's and I think Flacco is 40 now. I think there is a lot of runway for Fields to piece together how to play the position and be successful someday.
To temper that, I am not predicting he'll be MVP caliber or top of the league starter, but I think its a mistake given the great lack of supply at QB to think he'll never be able to succeed,