Yates has shown the form to beat Nibali in a Giro once and collapsed before winning said Giro. Nibali lost last year's Giro more due to tactical mistakes than due to age. You have guys who are pretty decent top 5 climbing references like Landa and MAL who weren't exactly flying away from him except for random seconds on more explosive Cat 2s.This was exactly the general sentiment before Giro 2016, which was considered Nibali's race to lose before it started.
He was much closer to his peak years and had probably the strongest team in the race. And yet he needed a miracle to pull it off.
Any of the riders mentioned could beat him. Yates in 2018 shape would beat him too. There's Vlasov who looks like the next big thing and possibly other young riders who will step up (Almeida?).