Network of mirrors?

Looking at where the reverse proxy caches are hosted, they are using similar method to various linux distributions for "live" data. They are relying on various hosting services to give them a server that they can use for the proxy server. I expect that any static data storage of the tiles would only go down ~15 levels deep before it got too expensive to store in a useful format.. and most people are probably wanting data at zoom level 17->20 versus those top 15.