You are correct, the megabytes are not consumed. The bandwidth though (MB/s of possible transmission speed) certainly is - it's an instantaneous resource - in this second, there's only so many megabytes that can pass through a single network link (be it a cell tower, fiber a optic cable, or whatever). And used or not, that data-transmitting capacity will be forever gone after the second is over. You can't use yesterday's unused bandwidth to transfer data today.
Peer to peer file sharing is inapplicable - it's a wonderful technology to allow data transfer loads to be distributed across multiple network links instead of all being bottle-necked by the source's single uplink, but if all of your nodes (including the source) are connected to the same wi-fi access point, you won't see any benefit. (peer to peer *networking* is a completely different concept, and not related to the topic at hand)
I think the issue is being confused because we're dealing with coincidentally identical units. Consider the physics example of energy(Joules) and torque(Nm). Basically unrelated concepts, and yet both have the same fundamental units, 1 J = 1kg (m/s)^2 = 1 Nm, they're simply written differently to avoid accidental confusion.
Similarly, we have two different concepts:
Data quantity, measured in MB
bandwidth consumption, measured in (MB/s of transmission speed) * (number of seconds it's used). = (MB/s)*s = MB
When a network provider charges you for 1000MB/s*s they are NOT charging you for the data you download, they're renting you the bandwidth you're using: Use 10MB/s of bandwidth for 100 seconds, and you've consumed 1000MB/s*s of bandwidth. That you've downloaded 1000MB of data is irrelevant to them, they'd charge you the same amount if you had consumed the bandwidth just sending a single byte back and forth a billion times.
As an alternative, they could charge you by the minute, just as they do with voice calls. But that would mean that if the network was congested, so that you were only getting 1MB/s, then it would cost you 10x as much to download the same file, because it would take 10x as long. That would certainly discourage using the network when it's congested, but I really doubt many customers would be happy with that arrangement.