Sure, it's possible but not convenient. Obviously phone carries do not want you to do this and are going to do anything they can to lock you into plans. Their current fear is being a 'dumb pipe' for wireless internet. This next level we're talking about basically makes them obsolete once wifi his a certain saturation.
1. You can buy phones from retailers who aren't carriers.
2. You can buy phones from carriers without signing up for a contract. If Sprint won't sell what you want to buy, then goto Verizon or another who will.
3. You don't need a phone to go wifi-only. This would work with almost any internet device, including tablets and laptops. The concept is the same.