They could MVNO (basically being a carrier reseller, buying bulk bandwidth from the actual network operator.) This is how Virgin Mobile, I think Boost, and some other networks have operated. Then reinforce with their own networks in places they care about (i.e. SFBA and NYC).
I don't think it makes sense, but I'm sure they've evaluated it. As a customer, having Apple as my single point of contact for everything mobile would be amazing, even if I had to pay a premium. Still wouldn't get around the limitations on spectrum and towers, though, but given that Apple's cash on hand is probably 15x Sprint's market capitalization, ...
