Youngster Kuss is only 4 years younger i believe, and i doubt Kuss will ever be a GT contender. Don't expect van Aert to take that step anytime soon either, maybe in a few years in case he 'd have won the classics he'd have liked to win. And even then. Foss is no GT material. Bennett clearly isn't either, Kruijswijk is a lot older, De Plus is moving away.
So it's only Roglic for the next 2 or 3 years, and that means he would always ride at least one GT as an undisputed leader, if he so chooses. Also, i think what we've seen from Roglic this TDF, is the best we'll ever see him, which is not the case for Dumoulin.