If you pretend that you're right and GMs are wrong, which is a pretty arrogant stance to take.
Look, this doesn't even have to do with whether you think Johnson is good or not. The point is that what you or I or any other fan individually think of Johnson doesn't mean jack ****, because fans have no say in roster decisions. The guys who make the roster decisions have shown a really clear trend on how they view Johnson, based on how he has been used, how he has been paid and what teams he has played for. You can look at those 3 and say that the Penguins could easily find a taker for Johnson if they wanted to, and have it be a completely fair claim to make.
He'd have to do even worse than what he has done so far for the Penguins to actually have a problem, because for as "bad" as Johnson has been over his career, he has been a top-4 staple for a decade, while playing on an Olympic team and multiple other national teams and making $50 million in contracts.