We might have a different outlook on what constitutes "constant contention". The Leafs won exactly one round in the playoffs while Dubas was GM. The construction of the team was faulty his entire stay there, and he never, ever addressed it.
Leafs have been at the top of the league for the entirety of Dubas’ tenure. They have an
average of 106 points per season in his tenure.
They happen to share a division with the only two teams more consistently elite over that period. Sucks for them.
Im not sure how Dubas gets the blame for any of their flame outs. In most cases their core players under-performed. The supporting cast he consistently found ways to replenish did its job.
At worst, his biggest mistake was being too loyal to the core 4, but not many GMs are going to look at a 60 goal scorer, a 100 point player, a 30 goal scorer, and a PPG center and say ‘this is the problem’. Hell, Im not sure Treliving moves any of them either.