Sirianni also doesn't call plays. Idk why DC isn't getting a pass for not having playoff success but you're still saying Ben Johnson is a historically good OC. Ben Johnson hasn't had any playoff success either by your definition.
Sirianni was an offensive coordinator when he got the Eagles job, whether he still calls plays or not.
I'm not saying a HC has to call plays, though in today's game it is an advantage to have one side of the ball protected from having its coordinator poached. I'm saying it's extremely rare to find someone like Campbell who is capable of going from a position coach to a head coach. So when the answer to every mention of a candidate's on-field performance is "but what about Dan Campbell," with the suggestion being that it doesn't matter, it ignores that the most likely path to finding a great HC is still hiring a top-performing coordinator.
We also have no idea how Campbell will respond to Johnson being hired away. I think they'll be fine, but a lot of opinions are resting on the last two seasons of having a historically-good offense.