There is no guarantee that if Wenger left we would be mid-table. Equally, there is no guarantee that if Wenger left, we wouldn't be better of. We simply have no way of knowing as the manager has been here for 20 years. There is nothing to compare it to.
One thing is known. Wenger as good as admitted he needs a striker when he activated Vardy's release clause. Regardless of whether you think Vardy is good enough or not, by making that bid, Wenger has said that he needs more up front. To get to September 1st without filling that position adequately would be a problem.