I've argued both sides of this point, but I'll re-attack with the opposite view.
Assuming a $92 million cap in 2025-2026...
Sign Marner 8 x $12.25 million
Sign Tavares 4 x 6.0 million
Sign Knies 3 x 3.5 million
Sign McCabe 4 x 4.75 million
Trade David Kampf (last year of contract)
Domi-Matthews-Marner
Knies-Addition-Nylander
McMann-Tavares-Cowan
Dewar-Minten or Quillan-Grebyonkin
Rielly-Tanev
OEL-McCabe
Benoit-Liljegren
Woll
Stolarz
Total Cap Hit: $85.6 million
That leaves you with $6.4 million to add a 2C that slides Tavares down the lineup. If you can trade for one retained, or find one at a cost effective price in the $4.0-$4.5, you can either use the difference to upgrade the roster elsewhere, or simply carry 21-22 players. You wouldn't be creating a lot of roster flexibility (unless you traded a guy like Liljegren as well) but you do free up enough money to add a quality player at center and it gives you a chance to get your young talent into the lineup.
I think the Leafs need to drop the approach of "can we fit Mitch Marner in and still have a decent team", in favour of, is Mitch Marner prepared to play in Toronto for a comparable amount of money to his peers around the league, when signing with good teams.
To do that, you have to ask yourself, who are his comparables around the league? what are they signing for? and are they going for max dollar (on likely bad teams)? or are they taking something that would be perceived as a discount.
The requirement should be for players to do as good, or better for the Leafs, than most other teams... and if a player's not willing to do that, then they need to be identified as a player you can't win with.
I look at a guy like Sebastien Aho, who signed for $9.75m on a first year cap-hit of $88m. 11.08% of the cap. Marner produces more, Aho scores more and is a centre. I think you could reasonably argue that they should be the same.
Another guy to look at is Barzal.... who signed for $9.15m on a first-year cap hit of $83.5m. 10.96% of the cap. Very, very similar players, with Barzal building an RFA year into the contract, so presumably his % should be a little lower than Marner's.
So next year we're looking at a $92m cap supposedly.
11% of that is $10.12m.
11.25% of that is $10.35m.
Call me crazy, but Mitch Marner's number should probably be $10.25m on an 8-year deal.
Should he be worth "more" on a shorter term deal given his current age? possibly, but the Leafs are in win-now mode...so they shouldn't be paying a premium to reduce the years.