That's the rub, if he thought about things logically rather than making a last ditch decision to move to Bournemouth I reckon he would have notched up a fair few minutes in all competitions for us so far. That's with hindsight in mind, as we've seen (bar the 3-match ban) Xhaka limited in appearances when I think we all thought he'd take the reins in midfield instantly although Wenger thinks otherwise. Le Coq, Ramsey and Elneny are all rotatable players too which makes for a midfield that Wenger can work with in different combinations throughout the season. It's not formulaic system by any means.
Santi was never going to play every week this season either and naturally Wilshere would occupy that position if Santi was absent. Even at a push, he could get minutes out wide as we've seen before. He's far from a limited player, a very Arsenal one at that, which has the tools to work across the midfield and advanced positions.
Or. Maybe he was told by Wenger that he'd have limited opportunities this season and that he'd have to take his chances, which, the possibility of picking up injuries coupled with the competent options he has to challenge against, maybe felt like they stacked against his favour.
Either way, I feel sorry for him and slightly disappointed at the same time.