ATT has wireline, IP-based network/Internet, U-verse, VOIP, entertainment, security services, etc ..., and so does Verizon. Their vast service infrastructures provide much better leverages and playing fields, which can not be undertaken by a mere TMUS nor S. Even the combined TMUS+S is still smaller than either T or VZ alone, in mobile wireless segment, compared to ATT or VZ.
~Market Value, Subscriber, and Employee stats:
T $251 B market value, 110 M subscribers, 246 K employees
VZ $175 B + Vodafone stake, 99 M , 176 K
TMUS $41 B, 34 M , 38 K
S $59 B, 56 M , 39 K
Let’s see who are in the deep pockets of the duopoly.