You have better chances of them asking JT to waive his nmc than you do marner leaving. Marner is guarded by JTs cap hit versus age. He's not first to go even if they want him gone.
They would never get rid of a 27 year old Superstar unless the trade is too good to turn down to the point hesitating is foolish. Almost every trade package for Marner doesn't meet this criteria.
If JT isn't asked to waive you can safely assume Marner is being re-signed just by using simple logic (or in a very unlikely event that 1 for 1 change of scenery superstar swap occurs). Logic also shows then that if Marner isn't a Leaf, it's both JT and Marner leaving. If you can't see why you're not qualified to be trading Marner in the first place. All in all, most avenues lead back to Marner retiring a Career Leaf.
It is far from as simple as saying "Marner is better & younger, with the same cap hit as JT, so they'll trade JT first".
The reality with Marner, is that they likely do not see a path forward in signing him. His negotiating range (barring him wanting to take a massive discount) likely starts at $100m, and stops at William Nylander's $92m.
Tavares on the other hand, there seems to be a belief that he's not as concerned about the moeny at this point, given his $77m contract which is coming to a conclusion, the $33m contract before that, his 33 year old age and 1100 games in this league over 15 years.
The Leafs have spent the last 6 years trying an approach with these 4 as the core and it hasn't worked. There seems to be a path forward where Tavares remains on this team in a reduced role and substantially reduced pay.
While I would argue that if Tavares goes down to $3m, and even if they give Marner a raise to $12m, this team COULD be successful with 3 high priced forwards and a continiously rising cap; I think that's probably 2-3 years away, and I don't think those 3 guys sanity will last that long.
There's also the consideration that Marner will probably fetch a pretty solid haul of a return, especially given his ability to be retained at 50%... whereas Tavares, not much of one.