Well yeah, staying behind Froome would have been Aru’s best bet, but easier said than done. The run-in to the final 500m climb was flat and twisty, and the pace was insane. Team Sunweb, BMC, Quickstep, and Dimension Data were pushing the pace hard for their potential stage winners Michael Matthews, Greg van Avermaet, Philippe Gilbert, and Edvald Boassen Hagen. These are guys who typically contest the spring classic one-day races. They’re stronger and faster than the GC contenders, and the teams above weren’t concerned about GC (except Quickstep for Dan Martin, but they were using their lead-out for Gilbert to do double duty for Martin, who’s more of a classics sort of guy than the rest of the GC contenders barring Uran). Point is, the skinny lightweights who are the main GC guys are not well suited to mixing it up at the front of the peleton when it’s doing close to 80kph. Every time you go around a corner you have to sprint back up to speed, and the power output is just beyond guys like Aru or Bardet. Power to weight they might excel at, but here we’re talking about raw sustained watts, and its the bigger guys who can do that. So if you’re Froome, you’re following a guy who’s making sure that he’s not leaving a gap behind him, and who knows that if he blows up it’s no big deal just swing wide and let the next guy work. If you’re Aru behind Froome, you’re following a guy who will actively try to exploit it if you lose a bike length.
Then you hit the bottom of the climb. This is 500m - a sprint, not a sustained climb. Aru’s superiority in w/kg is irrelevant. This is a finish for the puncheurs. It’s a peak w/kg thing. The huge watts the pure sprinters put out don’t help them, because their extra weight is too much to push up the 9% grade, the skinny mountain goats of the GC can’t dominate an anaerobic contest like this. Their superiority is aerobic. So it’s the Valverdes and van Avermaets who are going to blast up the hill the fastest. Most of the GC guys aren’t going to be able to quite keep up, though Uran on a good day and Dan Martin if he weren’t still hurting from Porte’s crash (and Valverde of course if he hadn’t crashed out) are the exceptions. But the climb is fast enough that drafting still helps, so again you want to be following someone who’s actively trying to keep you out of the wind. Everything is moving at a blinding pace, so any tiny misstep opens up a one second gap, and suddenly you’re five bike lengths back and don’t have time to recover.
Aru absolutely could and should have tried to hitch on to someone else’s train, but it’s not always so easy. Moving forward in the peleton when it’s moving that fast is hard for the little guys, and having a bigger, stronger teammate in front of you can make all the difference.
Now, if you’re strong enough this doesn’t matter. Uran had no help, and he was fine. But Uran is a guy with top ten results in races like Liege-Bastogne-Liege, Clasica de San Sebastian, and Strade Bianchi. Of the top 5 he’s by far the strongest in this sort of race (though Dan Martin is close and would ordinarily have a better finishing kick on something this steep). Martin also had Stybar pulling for him. Bardet is not suited to this classics-type riding, but was glued to Oliver Naesen and Jan Bakelants.