There are multi year contracts in place for the MVNOs, it would probably be very expensive for the major carriers to just shut them down. Additionally, I'm not 100% sure about the US, but in other countries there are legal and regulatory reasons that MVNOs exist.
If Verizon and AT&T decide they're going to drop subsidies and continue with the same pricing structure, we'll probably see the MVNOs start to disappear as their contracts expire.
And even if that DID happen, there is T-Mobile(and probably Sprint). Honestly I think their coverage sucks, but it's not too far from where AT&T was 10 years ago. If they had a huge influx of subscribers they'd have more cash to spend on expanding and upgrading their network. It's be similar to the situation AT&T was in about 5-7 years ago when the iPhone, iPhone 3G and then the iPhone 3Gs were released exclusively on their network. It took some time, but eventually AT&T put a lot of that money back into their own network.