Again, I have a hard time believing that when you look at the money spent. There's still a massive gulf between Arsenal and anything that is outside of the top6 in England. Spending close to 100m twice in 3 years isn't nothing, but you can definitely make an argument if it's needed or money wise spent as you say.
Xhaka and Mustafi. Neither of them have been excellent this season though, but I have hope that both of them become good enough players to become part of something bigger. IMHO they both fit the profile of players that can play key roles if the way we play is suited to their strengths.
The problem here is that as you say Wenger took ages to find exactly those two and in the meantime persisted with either not good enough players, injury prone players or straight up bought back-up that is no longer needed a season later, but not that we're not spending big money, which was my initial point.