The only other possible dark horse that could have emerged was either in US Cellular or Dish Network. However, I believe there were concerns with issues like market capitalization (in the case of US Cellular) or having different priorities (like Dish) that would have jeopardized a proper fourth option for the US.

I'm not suggesting the merger was "good" or anything like that. Just that the other options seemed quite unlikely.

As a Canadian (so possibly biased here), my own hypothesis for what would happen if a sufficiently-large long-tail power vacuum emerged in the US cellular data market — either back then or today — is that one of the major Canadian carriers would try to move in, beginning by serving cities just across the border from major Canadian cities.

You could easily do cell-tower-maintenance truck-rolls from offices in Vancouver BC to towers in Seattle or Portland; from Toronto to Buffalo (or, less plausibly, to Chicago); or from Montreal to Boston. And that's only if they even bother to operate towers — if they tried today, they could just as well operate as pure MVNOs.

In fact, flagship plans on Canadian carriers today, already usually build in no-cost full-speed US roaming data access through partnership with US carriers operating on the same frequency bands. It's a very short distance from there to operating an MVNO atop the same carrier's network.

(I would say that I'm surprised they haven't tried to do this already; but until recently, Canadian carriers were addicted to the extremely-high-profit-margin rate plans they built up through oligopolist price fixing. Our current government has seemingly broken that up for now, with much cheaper plans finally appearing — so they might finally decide it's time to expand their TAM to stay profitable.)

Dish is already making a hash out of its mobile customers (Ting, etc)

What's wrong with Ting?

Have you been a customer lately?

Yes, and haven't had issues with it recently... Was there some bad news I missed?

The reddit forum for Ting has a lot of complaints about billing issues, lack of eSIM support, and poor customer service (for the TMobile/DISH customers, of which I was one)

I'm a current Ting customer for the last ~5 years and have had no issues with them.

There is already a burgeoning dark horse competitor to the wireless companies: SpaceX. I believe within a few years Starlink will be a viable global wireless network.

They are using local partners

For now. But this is where the tech is heading:

https://www.pcmag.com/news/att-ast-spacemobile-promise-true-...

https://www.starlink.com/business/direct-to-cell

US Cellular is actually pretty probably, same tech stack, same hardware, similar customer bases, but they didnt have acres of capital. The issue was the albatross of the debt from the Nextel merger.