That was true in the past, but he isn't losing out now by continuing with his mission, compared to the alternatives. Yes he wants to take us back to the top flight, and partly because he wants the Teesside public to benefit from it (and partly because he wants to be revered for it again), but partly because it will raise the value of his asset substantially as well.
He'd be losing out more by selling the club now (or in reality writing off the debt/giving the club away for nowt).
I bet he wishes he'd sold the club a few years ago to be honest, but if he got a realistic offer now I think he'd be daft not to take it in truth. Rather than continue throwing good money after bad.