Here is the problem and its simply Math and making smart business decisions.
McDavid's deal 8 years @ $12.5 mil was the ceiling for all contracts as the best player in the game. (of which Edmonton purchased 4 UFA years in addition to 4 RFA)
Matthews deal should have been
8 years at $11.64 mil and not 5 years at that rate ( because 4 years where cost controllable RFA years and only 1 year was UFA).
Dubas claimed at time of signing that is all he could afford to for cap reasons, which suggest that each additional UFA year he would purchase was going to drive the AAV even higher, because he was getting played and was paying far too much for the 4 RFA years. In contrast Matthews RFA years at $11.64 mil was more than Tavares open market UFA years at $11 mil.
However the difference between McDavid @ $12.5 mil and Matthews at $11.64 mil is
only $860k extra.
So even if Dubas gave Matthews the exact same McDavid deal [8 years $100 mil] then he could have bought 3 more UFA years from a mere $860k more or basically the cost of league minimum player, and Leafs franchise player would have been locked up for 8 years not 5.
So now due to gross incompetence by Dubas the Leafs either risk losing their franchise player in a couple years for nothing if he leaves, and if he stays, his next contract is going to shatter all league norms coming off a 60 goal season and multiple Rocket trophies. So 20% C.H% CBA allowable per individual player at $83 mil cap ceiling =
$16.6 max.
So lets say Matthews gives the Leafs a home town discount of conservative
$15 mil per. Then if you then overlap it with the McDavid deal its;
5 years @ $11.64 mil + next 3 years @$15 mil for a total of 8 years = $58,195,000 current deal + $45,000,000 mil for next 3 = $103,195,000 mil [ more than McDavid $100,000,000 mill]
So Leafs will be paying
MORE for Matthews than McDavid over the same time period of 8 years, and what makes this GM incompetence even worse is if then re-signed Matthews new AAV goes up to $15 mil AAV,
$15 mil new AAV - $11.64 mil old AAV = +$3.36 mil more cap space extra (the additional 3 years) than had he got them for only $860k more had he given him $12.5 mil for 8.
Would you rather have McDavid for $12.5 mil AAV or Matthews for $15-16 mil per AAV for those same extra 3 years?
PS. Its the same
CAP HELL 
situation with Marner who should have been signed to the Draisaitl deal of 8 X $8.5 mil, instead of his $10.93 mil X 6 deal because of what his next contract will be.
If Leaf fans think Matthews + Marner + Tavares of 3 players making $33 mil is bad, wait to you see Matthew and Marner's next deals combined which are now likely to = ~ $30 AAV cap space or more perhaps all due to a greenhorn GM botching of all these contracts. Leafs are going to have to let Tavares walk just to use his $11 mil divided between re-signing Auston and Mitch top their new BIG raises.