There have always been riders who came close to winning a GT but ultimately never won one.
I'm not saying that Carthy is more unlikely than anyone else.
Just more unlikely than a chosen few.
Carthy is 26 and has not finished in the top5 in any big race apart from that Vuelta and I never trust the Vuelta as the ultimate measure of rider's potential. That's the first clue he might not necessarily be a GT-win level good. He is also not a great all-rounder, his positioning in the peloton is questionable, he responds poorly to accelerations, doesn't do well on explosive climbs, needs a race of attrition to excel.
Does he look more impressive than the best we've seen of riders like Landa, Lopez, Purito, Pinot, Kruijswijk and a few others who have never won a GT? I wouldn't say so. Thus why would I think that Carthy is the one who's going to win?
Also being 193 cm tall doesn't give him any advantages. I don't think there has been a GT winner over 190cm in modern era and that's probably not a coincidence.