How did Roglic win Catalunya, how did Pogacar win the 2020 Tour? Plenty of examples. There was another team taking control of the race, and all they had to do was follow. From the initial Giro team Jumbo announced, there are only two support riders that would make a real difference. Kelderman and Kuss. Kelderman has been out for months now, so they had ample time to find a solution for that or change their gameplan. The other is Kuss, and he's still there. Kelderman could genuinely have been a rider to get into the final with Roglic and do serious damage with a high pace, that could have put the entire Quickstep team in trouble. Foss however, is not that rider. As long as Roglic is able to make it to the final group of 20 riders along with Kuss, the addition of Gesink and Foss would be minimal, since their absence would simply put the responsibility to control the race, with QuickStep, UAE, Ineos... Gesink wouldn't make the final 20 riders, neither would Foss (or at least not with enough in the tank to mark moves or set a pace that puts rivals in trouble). And with other teams taking control, the actual importance of their absence should not be a deciding factor for Roglic.
As i said, there are scenario's like getting caught behind a crash or in echelons. But when Roglic genuinely gets dropped, those guys would be gone anyway. And when he wants to attack on the final climb, they would as well. The only real problem is if Roglic takes pink early and his "mediocre" team has to start working hard for 3 weeks straight.