If you don't get on Sprint / AT&T / Verizon / Tmobile store shelves you are DOA before you even think about appealing to users switching.
But something like Plasma Mobile does not need to care too much about that. If it has the technicals right, and can run Android apps, it can be an enthusiast platform nobody else adopts because Google has strong armed manufacturer control to keep that kind of competition off store shelves if they wanted to. Its also incredibly expensive to buy market share like that, something none of these participants have the money to do.
Since we took the opposite approach, focused on product first and demonstrated user traction and sales the stores and carriers came to us to talk and it changed the whole power dynamic.
Every year brings several new mobile brands to the market. Most you will never hear of because they're not exciting or special. They don't focus on making users excited about their unique product, rather they focus on business deals that even if they go through end up in no sales.
This might be so in US, but in most European countries we buy our phones ourselves with pre-paid options.
Also it sounds like your comment is technical point of view. Users dont care of that. If mobile system cannot attract regular users, then it is dead.