I think that if you go into a game with the menatlity of slowly slowly catchy monkey....it is hard to completely shift thinking in the course of a game whilst it is in progress. The opposite is also true where if you start a game gung-ho it can be difficult to suddenly switch to patience or the waiting game unless of course the team just incidentally gets tired out. The tactical shift is difficult mid game, not least because Wenger isn't a stand on the touchline shouty manager...... Well he screams at the 4th official but that isn't to change our tactical approach!
That is why there are a handful of managers of better clubs who make changes early and sometimes 2 subs at a time because of how difficult it is for a team to collectively change tack without any actual physical change happening. The subs coming on will be of a certain nature geared towards changing the tactical mindset and they will bellow out instructions to their team mates to ensure the tactical shift infiltrates the rest of the team. Wenger's first and most prominent instructions usually come at half time which is telling in itself.
Because Wenger favours energy and fitness levels over tactics in regards to his subs, his subs never impact games in quite the same way. When they do it is often simply because the fresh legs approach and/or the superior fitness levels of the team gets the better of the tiring opponents. The individual's he has at his disposal can also have a direct impact because of their specific skill set but it is quite hard the way we are set up as a team and organised for us to switch out tactics mid game for these reasons.