> So if all carriers provide only locked phones there may not be enough devices on the market.

Except you can buy directly from the phone manufacturer: https://play.google.com/store/devices/details/Nexus_4_8GB?id..., http://store.apple.com/us/browse/home/shop_iphone

Your argument basically amounts to: "if the carriers did have a monopoly on phones, then there would be a problem."