starter means whoever starts when all other elements are in place, ie none of them are injured or severely off form.
Which rarely happens, if ever. And you're not taking into account opposition, or previous or following games...
Quite clearly Pogba is a beast and is going to be a future great player, but if everyone is on form and fully fit in this hypothetical scenario right now I'd doubt you could say Pogba was a guaranteed starter, at least in Conte's mind. You certainly couldn't say it after 2-7 cameos he played, which is pretty much when our more whimsical members decided.
As has already been said; very different players, different roles, and eventually they will be alongside each other. The difference is that people will select Pogba based on potential and moments of form. Marchisio is not as naturally talented, especially in the physical sense, but he has already proven his role as part of our success.
It's no stretch of the imagination to say Pogba will be a lot stronger player than Marchisio over his career, but I do have to laugh about discarding Marchisio right now in comparison, especially when said people are just as likely to complain about lack of depth and competition at a later date.