I hope that doesn't happen. The Master was once offered a whole new set of regenerations, so I don't see why when we run out at 13 they couldn't do that. By that point we'll hopefully have had 3 seasons or so from each replacement, so it wouldn't seem totally out of place to have brought back Gallifrey by then.

Alternatively they might factor in some "Last of The Time Lords" thing whereby he can have as many regenerations as he wants.

I'm not sure which way out is worse frankly.