As we saw that the Datsyuk contract, even when a player retires, it doesn't not mean you can wish away his cap obligations. Yes, Marleau's contract was front loaded, but he will still carry a 6.25m cap hit in his final year.

Exactly right Pal, his contract is a +35 and as such his full cap hit would apply even if he retires! Best case scenario, is that Marleau waves his NMC, gets traded to a team struggling to get to the cap floor, and he decides to play the final year! Even after being traded Leafs could be on the hook for "cap recapture penalties" should he retire. (Only owed $1.2M in 2019/20)

I can also see Matthews getting a larger contract if he, at the minimum, stays at the same production rate. He's currently at 2.5 primary points per 60m. Compare that to McDavid's 2.6 primary points per 60m (which saw an increase from his rookie season, btw) and it seems like Matthews can land anywhere between 11-13m depending on how he does the following two seasons. He's bound to improve.

Good work on 2019-20. I'm trying to work out that year as well.
I've also been looking at 2020-21 and the issue seems to be the # of RFA's needing to be signed, like Liljegren.
Something will have to give I think.

One intangible that Marleau's contract could bring (if we are lucky) is a benchmark for future contract resingnings...now he is the highest paid player at 6.25M/yr and Lou can say look at his curriculum when other players have to sign....Nylander and Marner might then be signed to 6M x 6yr (would be heck of contract if 6M x 8yr), and Matthews as franchise player would get more, how about a Stamko's kind of deal at 8.5M/8yr.
The way Tampa and Yzerman worked out their roster IMO is the way other teams should try to work with their players to succeed, Matthews will get what he wants bcz there is not way around it, hopefully he sees potential for a great chance of success in Toronto and goes Stamko's way to help build a roster around him, but if not Leafs has no choice but to give him whatever it takes.
Nylander and Marner if want to break the jackpot maybe get a bridge deal and then one of them dealt like Drouin...I hope not the case and the 3 of them get reasonable cap hit to get competitive rosters the following decade.
I still believe 8yr for all are best option as second half of their contracts if cap ceiling goes up avg 1.5-2M/yr you get to 81-83M in 4yr, if the 3 take something like 10M+8M+8M or 26M cap hit still have 55-57M for roster or 2.75-2.85M avg/player for 20 spots, right now with 23 on roster (not counting Horton-Lupul) we are 2.973M/player avg salary.
I wouldn't be surprised if Leafs acquire some other Horton-Lupul type contract/s to play LTIR in future