I know how everyone loves to dump on the carriers but the "dumb pipe" idea is not feasible for a couple of different reasons.
The wireless network doesn't have practically "unlimited" bandwith like the internet. Because of this the carries are extremely interested in making sure that the devices running on their network do not consume excess bandwith.
Everyone phone that is made available for sale by the carriers is extensively tested to make sure it won't tank the wireless network.
The other issue is support. If you have everyone and their grandma using all variety of devices on your "dumb network" then how exactly do you train a customer service organization to support every phone known to man including one made in 1992?
The biggest reason carriers carry phones, is so they can limit the amount of trash on their network.