The US phone market is just going the way of the European phone market. You'll still be able to get a contract and subsidised handset if you want, but you can also get a SIM only deal and bring your own handset.
Not everyone can afford to drop Â£500 on a phone outright so there are many people who still go down the contract route.
The SIM only deals will be split into two. Either you top up the SIM at the beginning of the month and get a bunch of texts and data - or you can get a contract for your SIM which gives you a load of minutes, data and texts for a monthly fee.
Last time I had access to a network operators stats (4 years ago), customers on contract were about 51% of the total base. I wouldn't be surprised if SIM only is now the majority.