Your team usually doesn't win you the Tour in the mountains but it keeps you from losing it. Froome in his Sky heydays never would have had to react to the attack on the Telegraphe, he wouldn't have had to waste so much energy early in the stage and even if had still been on a bad day he would have been surrounded by so many teammates until late on the final climb that riders wouldn't have dared to attack. In 2017 Froome effectively lost 22 seconds in an uphill sprint in Peyragudes, but because nobody had dared to attack the mighty Sky until the final kilometre his bad day resulted in barely any time loss. Who knows if he even wins that Tour, had he been isolated earlier with Bardet, Uran and Aru attacking on the Port de Bales or the Peyresourde.
There are so many possible scenarios how Vingegaard could be saved by his team. An ill timed mechanical, a crash, maybe Pogacar attacks from so far out that even Vingegaard doesn't dare to follow and rather wants to rely on his team chasing. And then there is the possibility that Pogacar actually manages to drop Vingegaard. Just because a rider has been able to follow another riders attacks for two weeks doesn't mean he does in the third week. In 2015 Quintana was by far the best climber in the third week after getting demolished in the 2nd one. Yet Froome's mountain train kept him in that Tour.
Roglic and Kruijswijk abandoning was absolutely huge for this gc battle. TJV doesn't have the numbers to put a climber in the break anymore, WVA might be the best bike rider in the world, but he is still not a mountain goat who you can rely on near the end of every mountain stage and Kuss is infamously unreliable. If Vingegaard is isolated early he isn't just more vulnerable to Pogacar's attacks, a tactical move from a team like Ineos suddenly becomes a serious threat and a mechanical in the wrong moment can end all his hopes of victory at any moment.