What you are saying is "they won't be buying $700 phones for the kids if they get a bill that shows the phone costs $700 phone, instead of a monthly phone bill that hides the $700 payment".
They also won't buy $700 phones every two years when they don't have a contract where they have to _pay_ for a new phone every two years anyway, whether they buy it or not. With T-Mobile you paid for your phone after two years, so your bill goes down unless you want a new phone. With other carriers, your phone bill doesn't go down.
I'm saying they won't be buying $700 phones period if they have to pay that upfront. They will by the cheaper phones, just like they do in other countries where there are no subsidies. And I agree, the carriers were the ones benefiting when people didn't buy a new phone after their contract was up. They charged the same rate.