Just keep in mind that hitting 40 Mbps on AT&T and keeping that up for three and a half minutes will net you a gigabyte of data usage. That's $10 or $15 right there. The network is really fast because everyone has to watch their usage or pay a lot when they go over...and also because AT&T can sell themselves a ton of backhaul if they want, since they're the ILEC in Atlanta.
As for MVNOs taking/not taking a ton of bandwidth, until recently MVNOs coudln't get good smartphones on Sprint. You use more data when you have a phone that is worth using more data on, and phones like the Optimus V (as much as I like the OV) weren't the ticket.
As for how Sprint's adding capacity via Network Vision, let's put things into perspective. Its 3G network has around 10 Mbps of capacity, in real-world terms, on the downlink of any given 5x5 spectrum slice if it's all dedicated to EvDO (which, in many cases, it isn't...many cell sites have fewer EvDO carriers than that). LTE on that same spectrum is on the order of 35 Mbps, though 25-30 Mbps is probably more realistic. So spectrum is much more efficiently used here, even if Sprint didn't have a whole new band (PCS G) that they could use for LTE (so they're getting 30+ Mbps per sector without impacting capacity elsewhere). In many markets, Sprint has enough "spare" spectrum in PCS A-F that they could launch two LTE carriers (one in G, one in A-F) without issue, for 60+ Mbps of capacity per sector, compared with 10 Mbps or less on a 3G site (and that's assuming 3G had proper backhaul, which in many cases it didn't). And that's just on the PCS side. Throw in a 20MHz TDD-LTE channel on 2500MHz and you have another 60+ Mbps of capacity over a portion of the cell. Throw in a 5x5 LTE carrier on SMR and you have another 30 Mbps with better coverage. Granted, PCS is the only thing we're seeing right now, but even with PCS-only LTE (and the associated backhaul bear) Sprint has much more bandwidth to go around, allowing more people to experience better speeds without things slowing down to a crawl like they have on EvDO (except where I now live ).