What I don't understand is how any of them can be particularly flagged as competition for each other if they offer / impose roughly the same value, incentives, and penalties for the consumer.
Because despite offering "roughly" the same [whatever]
most of them offer something unique that makes them competitive for certain groups of people.
Sprint's "Simply Everything" plans include unlimited data/text + a voice plan. The cheaper plans (400/900 minutes a month) save around $30/month over the price of the same services on AT&T/Verizon. Sprint's $99/month plan is unlimited data/txt + unlimited voice. That's a huge value that sets Sprint apart from the other three.
And how many years did T-Mobile offer the value of the "faves" before another national carrier (Verizon) picked up on it?
VZW's dumped billions in rural coverage and roaming agreements, so if you're someone who travels the US a lot and needs a signal, their network coverage (esp in rural areas) can be a huge incentive.
Thank god AT&T has the iPhone, because I can't really think of much they're doing to stand out from the other carriers.
I don't disagree with you that a lot of what the carriers do doesn't differentiate them from the others, but there are a few things that each of them are doing that I think does make a huge difference (when it comes to values and incentives, anyways).