Miller is overrated IMO.
The talk about him is endless, yes he has the win in the rain at Assen, and the pole in Argentina where he took a massive gamble to get that pole. He couldn't convert into a win. Doesn't mean he isn't talented but he has no business on the factory Ducati. Petrucci whom I dislike for various reasons is a far better fit on that bike.
So why is Petrucci a better fit?
Yes I am biased but to me Miller has shown thus far in 2018 in his first year on the bike that he has enough over Petrucci to as a minimum be his equal which being the younger is a more attractive aspect for the factory.
Miller is no championship threat now or I suspect into the future but nor is Petrucci with both being quite close in the points with Miller on the 2017 and Petrucci on the 2018).
Yes Petrucci smiles a lot and has that funny little lilt in the accent but for mine he is as undeserving of a factory ride is Miller if one is to use career to date.
Sure Petrucci had a good 2017 with the inclememt weather podiums but one good season does not a rider make .