Actually, Sprint and T-Mobile are both on-record as planning on merging. They aren't planning to do so in the immediate future, but they both see the long-term end game being just three carriers in the entire US.
All they are waiting for is the DOJ to make a mistake and let it slip through.
"T-Mobile exeutives say Sprint merger the 'logical conclusion'" http://www.theverge.com/2013/9/25/4769794/t-mobile-executive...
"Sprint executives echo T-Mobile merger message" http://www.kansascity.com/2013/09/26/4509601/sprint-executiv...
At one point AT&T tried to buy T-Mobile, citing Verizon being bigger as why it was okay. They even put a bunch of money on the line if it fell through. Regulators decided that either they wanted 4 parties, or that Verizon would be forced to buy Sprint to compete, leaving 2 parties. (I am not 100% certain which they said was the reason)