When I go to virginmobileusa.com web site, I see phones such as LG101 which is a low end 2G/3G phone and HTC EVO V 4G which is latest high end (for VM USA) wi-max phone.
Virgin Mobile USA started with 2G/3G network and is expnding to future state (4G LTE). In betweek they want to utilize existing contract and capacity of wimax via Clearwire.
Whenever, there was something new to offer, VM USA came up with promotions and plans. People switched to new phones with new plans. SOme people stayed back on their 2G/3G phones and plans. With HTC EVO V, people switched to wimax 4G with plan rate increase (minimum 35 USD).
So VM USA still sees demand for basic 2G/3G phones and 2G/3G network and hence is maintaining and providing tjhose services/networks. If the demand for wi-max 4G, HTC EVO V keeps rising then Sprint will renew the contract with Clearwire (Sprint owns less than 50% stocks of Clearwire and has 7 out of 13 members on Clearwire board). Clearwire is building 4G LTE network and is also maintaining the wi-max network (until there are customers like Sprint and others).
We will have wi-max available the same way as we still have basic 2G/3G networks available. In fact, I read that Sprint and BVerizon are upgrading 3G network to EVDO Rev B (they do not want to rely too much on 4G demand/network - which is not rolled out to to each and every corner of USA).
In worst case, if wi-max and HTC EVO V 4G phone demand goes down at VM USA, we will have 4G LTE phone by 2015 and a new rate plan. Also, are we going to hang on to HTC EVO V 4G until 2015? We as customers also want to switch to latest and greatest as soon as possible.
Thus, it is all in demand and supply. We can enjoy HTC EVO V 4G and wi-max as long as there is demand for it. Think of today and now (the American way) and lets see what future 4G network Sprint/VM USA brings to us which we do not have control over.
Here is a good tutorial on 2G, 3G, 4G, etc.
2G, 3G, 4G, and everything in between: an Engadget wireless primer -- Engadget
I personally will prefer universal free (or fixed minimum monthly fee) seamless broadband wi-fi network across USA.