I'm sure the AFL could negotiate that one if needed. The AFL would be the MCG's biggest source of income and all the AFL has to do is schedule games away from the MCG during the season, only meet its minimum contractual requirements and I'm sure the MCG would lose more income than they would by trying to hold on to something that only happens once every 3 decades.I think after the 2010 replay the issue of interstate teams and having to travel again was raised by Demetriou. AD had the same view as you, that perhaps the replay could be played elsewhere. The MCC had a different view, however, and stated that their contractual agreement was for AFL grand finals to be played at the MCG. Their view was that if it's a GF, it would have to be at the G under the current contract. Would make for an interesting legal stoush if it were to ever happen.
Not only that, but even if it did happen, there would still be a 55% chance of the replay still being at the MCG if it was a Victorian side that finished higher on the ladder.