I think a couple things can be true.
COVID hurt the Leafs probably more than any other team. They banked on the cap going up. They signed Tavares and worked to keep AM, MM and WN under contract during the prime years of the Tavares contract with the understanding that by now, as a % of the cap those deals would largely be fine.
You can criticize Dubas for not adjusting the strategy but to be fair, I think they still made the best decisions they could. They can't move JT, they shouldn't move AM, and WN is under a bargain contract for the on-ice value he provides. You're left with Marner - and the question is, are you better trading Marner and reallocating his cap space or keeping him? Based on what I expect the return would be, I would bet that the on-ice value Marner provides is better.
Its not as simple as "well, adjust" when the adjustments available are worse. Dubas built teams that set franchise records in points, the idea the result would have been different just by offloading Marner or Nylander for something else, while possible, to me isn't any more likely than running it back.
I think Pittsburgh is in good hands - Dubas is an above average GM in this league who is only going to get better his second time around. I think the task in Pittsburgh is much harder, but i'm excited to see how he puts his stamp on that roster.