125M EUR is a fantastic figure but it's worth it for Wirtz as long as he performs similarly at Leverkusen.
It is possible that bringing in Wirtz will kick Havertz out because this player is not so impressive while at Arsenal or indeed as you say Arteta does not want when there is a problem of attacking line injuries lack of players so before that happens it is better to bring in players first, in addition to the current schedule.
Still asking if Arsenal will give so much money for this one player? Arteta's desire seems clear that Wirtz is a great player and fits at Arsenal.
I agree that it's a huge amount, but also Wirtz definitely deserves that kind of value as well. Think about it, this guy is the best player of a team that ended the 10+ year run of Bayern Munich, that means he definitely worths a lot of money, and I think there could be some teams who will want to get someone like him as well, I do not think that we are going to see anyone changing a team as much as him for that price range so I think it should be a fine deal.
I think if KDB goes to Saudi league, which there are talks of for next summer, then maybe City could think about a move like this. I know even though Wirtz is an awesome and young star, he will not be as good as KDB, but he can at least cover that loss a bit and could help them.