I'm not sure tagging Darnold would hurt their 'good guy' standing. I mean, if they trade him, he'll be gone and the rest of the players surely would prefer to have the better draft capital to try to build a better team.
I think it comes down to what we've all been worried about since the playoff game. Will they get stuck with Darnold if they tag him? I still think that's a significant possibility. While he's probably the consensus top QB available, I don't think he's the unanimous top QB available. He indisputably has the most upside, but Rodgers, Cousins and Wilson probably have a much safer floor. And most of the teams looking for a QB, are probably looking for a bridge QB; as a Vikings fan, I wouldn't want ANY of these guys long term. The safer, cheaper, older QBs may be more attractive to the teams that actually need a QB for just a year or two but still want to see if they can squeak into the playoffs.
Put ourselves in the shoes of a Raiders or Giants fan. As a team that's not close to a championship, do I want to give Darnold 40M for three or four years? Or give Rodgers/Cousins/Wilson 20M for a year or two. I'd go for the second option in a heartbeat. Because while Darnold has the higher ceiling, it's not high enough to make you a legit contender.