My guess is this is the Trent Dilfer story. Trent won a Super Bowl with the Ravens, but everyone thought that he was easily upgraded and would be a long term liability so they cut him and signed Elvis Grbac (I think I have this right.) Then, they signed another QB and another QB, and suddenly this team fell out of the playoffs and into mediocrity.
Sometimes, a QB just clicks with a team and getting rid of them to get a better QB sets them backwards. Frankly, I think that happened with the Vikes when they signed Kirk. The year before Kirk's signing they were in the NFC Championship game. They thought their journeyman QB was holding them back so they went for broke and signed Cousins. The Vikings got worse results after the signing. Part of that is because of the players they couldn't afford to retain because of Kirk's salary and part of it was because Kirk just isn't the QB his stats say he is. In any case, Darnold represents both a danger and a promise.
He may return to the mean or he may just be the right QB for that team and with them everything clicked.