Those levels are so arbitrary, like, "hey, let's just increase the requirements a little from the current while getting rid of the lowest level! Brilliant!"

You need the lowest level to give low to moderate flyers an incentive to stay with united. They won't otherwise, and airlines realize that. I wouldnt be surprised of silvers made up half of the elite ranks.

I do think it would be interesting if they put in some sort of a spend component, liked a tiered spend benefit, apart from the mileage requirements. They would have to be fairly high, though, like minimum $15-20k or something.