I disagree, I think Marsh's performances in League 2 last season give us an idea as to what we could expect from him in the Championship, just as Cannon's performances last season (and in previous seasons for other clubs) can.
Those assumptions can be wrong. Marsh could take to it immediately and look good at this level, just as Cannon has turned up and not played well despite having a decent amount of game time.
It annoys me that people suggest that it's an obvious decision to drop the senior striker with pedigree to play the unproven kid. I'd be excited if it happened, and wouldn't complain, but to suggest that the manager is compromised if he doesn't do it is moronic.
Had Barry played well against Watford, I'm sure he'd have started again, he was picked over Cannon only a few days ago. Eventually, enough players will pick up enough form and fitness to force Cannon out unless his form improves. The hate from some (not you) is infectious and unhelpful.