I disagree. 1st. tier cellphone companies DO in fact have to be big .... The dollar amounts involved to roll out and maintain a cellular network across a whole country the size of the United States is steep enough that the little guys just can't accomplish it well.
What we do have room for are the 2nd. tier "regional carriers" -- and personally, I'm disappointed we haven't really seen more happening in that arena. If you're not big enough to compete with the likes of Verizon or AT&T in nationwide coverage, fine. How about focusing on providing top quality coverage and customer service, with good data performance, all within a few states?
For many years, I had an account with U.S. Cellular, in St. Louis, Missouri, and was very pleased with them. Their little marketing strategy of "all incoming calls are free" meant I didn't really need to buy a lot of cellular minutes on my plan. (It's relatively rare I place a call to someone vs. all the times I'm taking a call.) Signal strength and call quality were excellent too. Really, the only downside was a relative lack of choices in phones, because you had to select one designed to work on their network - and they didn't have as much pull as the top carriers to get the latest handsets first. Still, they'd typically manage to get at least 1 or 2 of the "hot" phones out there at any given time. (I had a Motorola Razr flip phone with them, when it was still the in thing.)
T-Mobile, IMO, has really gotten on a roll with upgrading its network to become something respectable. It has a lot of issues still, but as a current customer, I see evidence all the time that change is happening. (My phone has carrier updates pushed to it practically every week, as new towers come online.) Just last week, something changed where I live, too. For a couple days, all of us received "no service" or weak signals throughout the business day, but then suddenly, things came back up with a signal strength far superior to what we ever had before. (I used to use a signal booster in the house, but was able to turn it off after the upgrade.) Can't say if it was a new tower, or a modification or repair made to some existing one -- but it was a nice improvement.