Ultimately, the team was doomed when Dubas got rolled in contract negotiations on the Big Three's second contracts. Plenty of other big mistakes like signing Marleau, trading Kadri, losing Hyman, not finding a legitimate starter, etc. But it all comes back to not having any cap maneuverability because you signed three RFAs to ludicrous deals that were both too expensive and not long. It forced a terrible Marleau trade to make cap room, it cost us Hyman, it meant never being able to fully flesh out the defence for years, it meant never being able to afford quality depth players and heading into the playoffs with a patchwork bottom six every time.
There was zero reason to pay Matthews more than Tavares on a shorter deal and as an RFA. No reason to drag Nylander's negotiation to the eleventh hour if you were going to cave anyway, and no reason to give Marner a more than $10 million AAV. Absolutely ridiculous, compounded by a frozen cap. The Leafs could've and honestly should've pivoted off of this financial model, especially after the 2021 debacle, but had those contracts been more reasonable the situation wouldn't have been as untenable as it was.
Those contracts ruined the culture and cost us heart and soul guys that should've been Leafs over the long-haul.