This is exactly what you can't do. Hope is a lousy contingency plan.
You basically have to treat Leary like you would an aging vet: assume he has a limited shelf life and will need replacing at any time. You might get lucky. He might play for ten years without the knee affecting him. You just can't plan for that.
Which is why, as good as he's been, you can't neatly lump him in with the three first-rounders in discussing the future of this O-line. His youth is, in a very real sense, potentially misleading.
Of course, he has nevertheless more than justified the investment the team made in him as a UDFA. Still, investing a draft pick in him would have been questionable given that he cannot be counted on (such that a draft pick would not adequately address the need without the further expenditure of resources needed to put the contingency plan in place).