It seems to me Wenger has become (too) risk averse.
He's always trying to avoid overpaying - and ends up underbidding; so we miss out... or, he convinces himself that what we have is adequate - when evidence proves that it isn't.
Something needs to change as we're risking another failed transfer window... and yes, Xhaka is a good signing - but as the man himself said at the end of another season where we failed to maximise our opportunities, we need three or four quality players. Not one.
What needs to happen is Wenger identifies the player/s - and someone else negotiates that deal. We need another DD, but that won't happen.