Only scenarios that Marleau won't be on the roster for his final year are a) Leafs win the Cup this year b) career ending injury.
You should count on him playing next year.
That is true, but for clarity for others, who might not know the CBA rules.
Even if the Leafs won the Cup and Marleau then retires a happy man, the Leafs still have his $6.25 mil on the Cap regardless as it was a contract signed after age +35 and there are CBA rules for that in place. If he did retire after winning Marleau would be leaving $4.25 mil on the table as lost wages (his salary in year #3), so it would be unlikely he would forgo that.
He has a full no movement clause /no trade clause and couldn't be dealt or waived without his approval. So it would be up to Marleau entirely as to staying or going, Leafs having no options here as even a buyout would result in the same full cap hit.
If he were injured and couldn't play then Leafs could put him on LTIR and get a credit/reimbursement for his cap cost and replace him with a healthy player at similar or less cap hit. ex If Marleau were hurt his $6.25 mil would still be on the books, but if Leafs resigned Nylander and he got $6.25 mil or less than they could recapture Marleau LTIR salary and use it to cover Nylanders cap hit, essentially overlapping and only 1 player counting against the cap. Marleau @ $6.25 mil (LTIR) + Nylander $6.25 mil =
only $6.25 mil total against the cap not $12.5 mil.
But the most important thing that many Leaf fans keep forgetting and this is related to the above LTIR (long term injury replacement) CBA situation is that Leafs have the injured Nathan Horton on the books at $5.3 mil and on LTIR.. So Marleau makes
$6.25 mil - Horton LTIR $5.3 mil =
+$950k. So essential for Leafs salary cap calculation purposes
its only the difference of $950k that Leafs need to accommodate below the hard cap ceiling (Currently $80 mil). That balance of the LTIR reimbursement ($5.3 mil) can be above the upper salary cap limit. Hard Cap of $80 mil + Horton at $5.3 mil LTIR = Leafs spending of $85.3 mil salary cap.
So in year #3 with Marleau playing he only costs the Leafs similar cap hit to a Marlie player making $925k (like Grundstrom, Rosen, Ozhiganov,Sandin, etc etc).
IF the Leafs don't win the Cup this year, you can 100% believe Marleau will return next year to honour his contract, even if it were in a reduced role to take one last shot at it in 2019-20. IMO