@Alex Probably a big ask, but do you have the statistics of how many nations delete at each city level? I'd guess based off of Sketchy's war stats thread the vast majority is at city 1 or 2, but dunno if you have more exact numbers over a larger period of time that could show where exactly players are leaving after sticking around past that initial creation.
Right now I'm on Bourhann's side of the argument. The retention of new nations has always been a struggle for mass recruiting alliances. It's on the alliance themselves to keep players engaged - whether that be through an off site community worth sticking around for or frequent wars in game.