Honestly I find cycling so much harder to predict for a number of reasons, and obviously Clinical stuff is part of those reasons, as let's say sometimes a team just overperforms for a stretch of time. In tennis I feel like I can quite easily spot which players are gonna go somewhat far and which aren't just based on technique and player stats.This is such an interesting point. When Froome, Contador, Nibali and Quintana were all pretty much peaking at the same time (or with Contador coming towards the end of his peak) Aru looked like the logical successor. Who else was there at the time? I guess Quintana was still young and we couldn't have known that his peak was gonna be so short. Pinot and Bardet were never quite on gc winning level, Dumoulin and Thomas weren't even seen as future gc contenders at the time and the next big talents weren't on the horizon yet. The future was looking so bright for Aru.
Thinking of it, the weird thing is that if Aru had actually kept improving for just one more year going into 2016 there would have been a lot of very winnable gt's for him. It's not like the "big 3" situation in Tennis where Djokovic, Federer and Nadal simply had a much longer peak than anyone anticipated. Aru won his Vuelta in 2015 a year that arguably represents the end of the respective peaks of Nibali, Quintana and Contador (you might argue that Nibali's and Quintana's peaks lasted for another year but whatever) but Aru just couldn't capitalize because his peak ended right then and there too.
In that vein you might argue Aru was a bit of a one trick Unipuerto pony maybe due to his obsession with be so skinny etc. So his upside was rather thin, and I think there wasn't that much to indicate he was gonna lay waste to say Quintana and Froome in the Tour. And even in the 2015 Tour you had Nibali cranking out a 60km mountain solo like Aru couldn't dream of, and in that 2014 Giro and Vuelta he did get put firmly into place in the big mountain stages.
But the short window where he was good is by far the biggest reason he is hard to place. I struggle to think of even a similar rider other than Simon Yates maybe?
Also I can't really speak for the perception of Dumoulin cause Dutch bias, but IMO Dumo was always gonna be a bit of an all or nothing case that ended up closer to all than nothing in 2017/2018.