Did not read the entire thread.
But by the time Marchement was impressing with the Marlies, the Leafs were already well committed into their strategy...the Big 5...and tinker with the roster by signing veterans to help up front, or on dee, as the winds blew. The priority to their young players, and seeing if they could become reliable NHL players, was relatively low, and remains low. Compared to the league at large.
They felt they had the core to challenge for a Cup, and their time and resources were spent figuring out how to try and maximize their chances through impactful veteran acquisitions, off-season and deadline.
That strategy has persisted, with some exceptions. Marchement never had a ghost of a chance to show his stuff, given the others already ahead of him in the slim-chance sweepstakes for unproven Leafs forwards at the time.
Is it wrong? Not hugely. The Leafs rosters only had so much room to give try-outs to players like him, and rightly so. That they could not see anything of what he has since become? Well...therein lies the big question...but hard not give Leafs management a big fat F for this oversight, because as soon as he left, he became a reliable, plus-contributing NHL-level forward in every situation he was given.
It happens.