Just to riff on this - have any of the 3 QBs Canada has had reached anything like their ceiling here?
We know Trubisky is no great shakes, but he did manage to get to a Pro Bowl and was attracting rave reviews from an org that knows a thing or two about QBs in Buffalo. He can clearly play better than he did in Pittsburgh.
Post-injury Big Ben wasn't all that but he was a still a guy with HoF traits and the difference between his record when throwing on the Canada plan and his record when playing hurry-up sketching down plays football was stark.
We don't have a long list of Steelers offensive starters who had major workloads under Canada and what they were like elsewhere. Claypool and DJ were the only guys in the weapons room to play a bunch before Canada; Claypool went from a 9 TD rookie season to having half a foot out of the league after a season and a half of Canada. Lots went into that, but still startling. DJ, well, he now owns a record for incompetence. OL wise, the main example I think is Kevin Dotson, who went from "the guy who'll get Pickett killed" to "one of the best guards in the league".
Pretty much the only guy who has hit his potential as an offensive player here over a prolonged period is Jaylen Warren.
Throw in all the stats pointing to historic ineptitude, and a generally lousy career outside one bright spell at Pitt, and I really don't think it's far fetched that Canada was really that bad that he was that limiting.
I mean, what's there to argue for otherwise other than a belief no coach can have that great an effect?