I think a lot will or should depend on what the Vikings do in the playoffs. If they made a SB or deep playoff run then it's hard to move on from Darnold. If they manage to win it all then you have to bring him back. If it's a quick exit then I think you move forward with McCarthy just as they planned.
This isn't too different from the Kirk discussion other than age/injury. There were a lot of people saying they shouldn't move on because he's a known commodity and he put up big regular season numbers. Vikings decided they'd rather have that young QB on a rookie contract and believed they'd be fine moving forward due to the coach, weapons, offense, etc. Kirk had his career years in this environment and now Darnold is doing the same. The main reason behind it is KOC, who handpicked McCarthy to be his QB of the future. So if you believe in KOC, which I do, then move forward with his guy and surround him with more talent due to the benefit of the rookie contract.
Best case scenario is winning the SB and running it back. Otherwise, tag and trade Darnold and have McCarthy start next season. Darnold's play might have been the decision more difficult but they shouldn't deviate from their plan years in the making.