The thing is, are TdG's numbers any better than, say, Rui Costa? Mind you, a WC, with World Tour 1 week GC wins. Or van Garderen? And look at how "little" van Garderen has won.
Rick has a point, in that de Gendt is good enough all-around to be a potential GC rider. His ride on strava has an average 350W for 5h, for Christ's sake. However, I don't think he climbs good enough, or TT's well enough, to be a real, regular podium contender. He's mentioned focusing on GC's in the past, after his podium in the Giro, but not really working out. As it is, he has seemingly found his own niche, and is the sort of rider appreciated by cycling fans all around the world. It's not clear to me how that's any worse than struggling between 7th and 13th in GC's (and these are all outliers)