The unwritten rules are a beautiful part of the sport. I'd argue the group shouldn't stop if a guy crashes, because that's 100% on his skills. If the leader has a mechanical or another type of emergency (bathroom break), in my opinion, the other GC riders should wait for him. Aru's was a cheap shot, but he didn't profit at all from the attack. He slowed down and let the group catch him again, and actually ended up losing seconds to Froome.
Now let's look at Nairo's and Nibali's attack to Tom during the Giro:
At the end of Stage 15, the GC looked like this
Code:Classifica generale dopo la 15ª tappa 1 Tom Dumoulin (Ola) Team Sunweb in 63h48'08" 2 Nairo Quintana (Col) Movistar Team a 2'41" 3 Thibaut Pinot (Fra) FDJ.fr a 3'21" 4 Vincenzo Nibali (Ita) Bahrain-Merida a 3'40" 5 Ilnur Zakarin (Rus) Katusha-Alpecin a 4'24"
During Stage 16, Tom had the bathroom break and the peloton attacked while he was out. The arrival of that stage was
Code:Ordine d'arrivo 16ª tappa: Rovetta-Bormio 1 Vincenzo Nibali (Ita) Bahrain-Merida in 6h24'22" 2 Mikel Landa (Spa) Team Sky s.t. 3 Nairo Quintana (Col) Movistar Team a 12" 4 Domenico Pozzovivo (Ita) Ag2r La Mondiale a 24" 5 Ilnur Zakarin (Rus) Katusha-Alpecin a 34"
Which changed the GC to
Code:1 Tom Dumoulin (Ola) Team Sunweb 2 Nairo Quintana (Col) Movistar Team a 31" 3 Vincenzo Nibali (Ita) Bahrain-Merida a 1'12" 3 Thibaut Pinot (Fra) FDJ.fr a 2'38" 5 Ilnur Zakarin (Rus) Katusha-Alpecin a 2'40"
Now that was an unfair attack. Had Nairo and Nibali not attacked Tom while he was out, he would've kept the pink for the rest of the Giro, which he deserved for being the best rider. During that stage, Nairo took 2'10" back from Dumoulin, and not because he was the better rider, but because Tom had to stop.
This is why I believe the unwritten rules are good. The overall winner is supposed to be the best overall rider of the peloton, not the opportunistic asshole who attacked while his rival was down, due to something out of his control.
Can't believe we're still talking about this. Why would anyone stop and wait for Dumoulin in this case? Because he had a sick tummy? What if he had sick legs too, should we wait?
His body responded badly to the stage and he paid for it. That's exactly what cycling is about.