I think if Darnold holds the job all year, it'd be an unprecedented surprise, and/or an indictment on McCarthy. I think Darnold being an upgrade on Mullens is a legit question.
I don't really see any similarity between Darnold and Mayfield going into last year. Mayfield had been a good QB for most of his Browns career, up until the shoulder injury. He also didn't have a high 1st round pick breathing down his neck in Tampa. I'm honestly struggling to even think of a similar comp to Darnold suddenly turning it on after showing next to nothing for 6 years. Maybe Geno Smith, but Geno doesn't get that job if he doesn't have that great month (FAR better than Darnold has ever been) when Russ was out, and even then, needed a team to give up a ton to trade for Russ so there was no competition.
Are there any busts who signed with new teams and turned their career around while holding off a top pick in history? Not decent QBs who suddenly started playing better (like Tannehill) but someone like Darnold who has never shown anything.
Chris Chandler in 1995 holding off Steve McNair is probably the best, but McNair was a package QB, who was a unique case.
Jon Kitna in 2003 holding off Carson Palmer is probably pushing it, as Kitna had led the 1999 Seahawks to the playoffs.
Lynn Dickey in 1981 held off Rich Campbell and became the Packers starter for the next several years, but he'd been on the team for 4 years by that point.
If Mitch Trubisky had suddenly started playing well in Pittsburgh, and held off Pickett, I think that would have been a similar situation, but that didn't happen, and even Trubisky had a division title to his name. Historically speaking, there is zero reason to expect Darnold to be anything other than the mediocre backup caliber QB he's always been, with maybe a 1-50 chance he's another Chris Chandler. The most likely scenario is the Trubisky Pittsburgh one, and hopefully for Vikings fans, McCarthy is a lot better than Kenny Pickett.