And what have you seen to lead you to believe Fields is. The majority of all 1st rd QBs fail to become that SB caliber player so the odds of Fields becoming one are against you.
You could make the same argument about pretty much any position. Andrew Luck was supposed to be a generational talent and yet the Colts never really went anywhere with him. There are few single players that can make that much of an immediate impact regardless of position.
So I'm new to all this and really don't get into all the trading what ifs, but if Lance is there at 8, wouldn't you at least think about taking him instead of trading - regardless of Darnold.