I think we should be looking at it another way. There's no need to write the season off yet, we are still in the mix for promotion, possibly automatically, but more realistically through the play offs.
However, I do agree with you that we should be planning now for Johnson's replacement, irrespective of whether promotion is achieved or not. If he can't set a side up to keep clean sheets in this division, he's got no chance if we go up.
The search for his replacement should begin now and when that person is available to take over (in the summer or sooner), then Johnson is shown the door.