The Grant of Rights isn't to ESPN...it is to the conference. The conference is the receiver of that grant and would decide whether to enforce it, or not. I suspect nobody in any GOR conference wants a challenge that may affect their GOR,
A school grants the conference the right to negotiate the contract. Without a GoR in place, ESPN would not know for sure what it is getting in this day of realignment. With the GoR, ESPN knows exactly what it is purchasing, even if a team changes conference. And the price ESPN is willing to pay will rise accordingly.
Once ESPN has acquired the rights via the conference and agreed to a price, it gets interesting if a team moves to another conference where ESPN also holds the rights. ESPN has what it bargained for, no damage done there. And as long as the former conference is still receiving the agreed upon payout, hard to show any damage there. The conference receiving the team now has an extra mouth to feed, but if said conference is already in the process of negotiating a new contract then there can still be plenty of cash to go around.
ESPN is then free to pick up whatever game it likes as Tier 1, and the conferences don't care so much because the payout for the season was already determined. Also, ESPN has to walk a fine line here with the conferences because there is probably some sort of "best interest" clause in the TV contracts, so they would want to keep both of their customers happy.
BTN might be a little upset if games don't fall to them because of the GoR, but I think the impact would be relatively small given that there would be other games available for back fill.