People work on their mobile devices at the annual Consumer Electronics Show (CES) in Las Vegas, Nevada January 8, 2014. REUTERS/Robert GalbraithBy Sinead Carew NEW YORK (Reuters) - New Year's rivalry among U.S. mobile operators has Wall Street worried that the industry's profits could seriously decline. After months of aggressive moves by T-Mobile US to lure customers from other carriers, No. 2 operator AT&T Inc counter-attacked on January 3 by offering to pay consumers to switch from T-Mobile. On Wednesday, T-Mobile upped the ante, saying it would pay hefty exit costs for converts. The moves by Sprint and AT&T come after No. 4 U.S. operator T-Mobile, a long-time industry straggler, was able to report three full quarters of customer growth after four years of losses.