I'm not betting on McDavid reaching free agency, but what do you mean the team won't let him go? There's nothing they can do if he wants to go to free agency at the end of next year and every team in the league will be willing to make him the highest paid player in the league.
My odds are something like:
-80% chance he re-signs as an Oiler this offseason
-10% chance he re-signs as an Oiler in-season
- 5% chance he signs with the Leafs as a UFA
- 5% chance he signs with a different team in UFA
Meaning, if he becomes a UFA, i think the Leafs could be the favourites to sign him (if they have capspace); however, i don't think he's likely to become a UFA. It's something that should at least be considered when signing free agents this year IF Marner is gone given theres no similar calibre players available this year anymore.
If Marner walks, I'd rather sign guys to overpayment 1 year deals (i.e. Bertuzzi and Domi v1) rather then slightly cheaper multi year deals that won't age well and will limit our options in free agency next year (which CURRENTLY includes quite a few elite forwards and a lot more top 6 talent).