Unpopular opinion: the 'overpayments' of Nylander, Matthews, and Marner will cost the Leafs 1 quality piece that they will have to trade and (*gasp*) acquire picks/prospects that they can use to restock the system.
After the Hawks won the cup in 2010, they had to move Buff and Ladd for cap constraints. They were still able to retool the roster, stay competitive, and win again in 2013.
After the 2013 cup, they had to move Bolland for similar reasons. They were able to retool and win again in 2015.
The common theme here is that the Hawks had their elite core in place and had to find the right surrounding pieces. The Leafs are one contract away from locking up their elite core (arguably two if you include a Muzzin extension).
What will keep the team competitive is the ability to move supporting players for assets and turn those assets into cheaper supporting players.
I'm not a fan of the Matthews' deal, the term is too short for the AAV, but we were all expecting that sort of AAV anyway. The main gripe should be with the term. Nylander is also high IMO, and I expect Marner's deal to be high based on Matthews. But the criticism here is largely overblown in the big picture.