T-mobile's sell was offering no-contract service and having a network that you could buy broadly available GSM phones for (where with Sprint, you to switch carriers, you had to throw your phone away).
I really hope the merger will be more a mass transfer of Sprint customers to T-Mobile (and the existing T-mobile approach) but it could well involve the opposite; A big accumulation of customers could wind-up be squeezed for all they have as the combined effort is run into the ground. (that is a strategy that yield cash for investors, it's not more "irrational" than spending a bunch for growth, indeed, "build monopoly with low margins, exploit monopoly with high margins, little service and little capital investment" is natural for many companies and industries.)
I mean, keep in mind the two carriers have incompatible networks, incompatible phones and so-forth so soon or later, there will have to be a "reconning" where one ex-carrier's phones become useless, yay!.