It's this thinking that keeps us from losing sleep over the US falling into a Canadian-like situation of only 3 major wireless carriers, but you can't say it can't happen. Sprint's parent company does have deep pockets and could make an offer T-Mobile (and their stockholders) can't refuse. All concern about losing one of our carriers goes out the window when there's money to be made. Additionally, increases in interest rates have made the cost of money greater, making a big deal just a bit less likely.
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Some of T-Mobile's recent success (and improved coverage) has been built from the $4 billion of cash and licenses it got from AT&T as a breakup fee when AT&T's proposed acquisition of T-Mobile was blocked in 2011. T-Mobile would be smart to avoid that scenario by staying away from a merger of another wireless carrier. They don't want me to lose sleep, do they?