We can debate Fields vs Young all day as prospects/players. I think they clearly have their comparative strengths/weaknesses.
But perhaps the biggest point in the favor of Young is simply how building around him would work compared to building around Fields has worked so far.
With Fields we are entering year 3, we haven't had a 1st rounder (other than Fields) in 4-years. We've invested no money whatsoever in Fields supporting cast. We also had to trade up to get him which is not ideal (although still obviously a great trade).
It's much more Pace's fault than Poles, but bottom line this has been a pretty terrible attempt at building around a promising young QB. This offseason will be the first of Fields career where we can actually make strides towards meaningfully improving the roster. And as a result of that, we're not going to be able to build up a playoff level supporting cast before Fields is due an extension which is flatly a failure. None of this is Fields's fault by the way, but it somewhat works against his 'value'.
Compare that to building fresh around Bryce Young, and it's a completely different story. If Fields is traded for 1-2 first rounders, then we'd have 4-5 first rounders to put on the roster during Bryce's rookie deal. As we know, we have the most cap space in the league to purchase a few quality starters. We can realistically have 4 straight offseasons of improving the roster that line up directly with Bryce Young's rookie deal. That's very different than for Fields where we essentially wasted 2 offseasons.
Again, the above has nothing to do with comparative worth/abilities of Fields/Young. But as an exercise in team building, it's pretty clear which path has the upper hand.