He left a decent core. They hired the wrong guy to fill out the rest of the team
Core has demonstrated they are not the group to build around for post season success, or at least not all of the core four at their significant proportion of the cap.
Good in the the regular season but abysmal in playoffs isn’t “decent” in terms of building a contender.
When he interviewed I suspect Treliving indicated he wanted to trade some of the core four but there was a top down instruction to give them one more year first.
The Leafs played well falling short as they don’t have the depth of personnel of a normal contender given cap constraints.
Some of his new recruits performed well, if they can get Marner and Tavares’ caps off the books that opens up a lot of possibilities to develop a legitimate playoff contender.