Yeah, there's no way people would want a car that hits the speed limit in 4 seconds when there's cars that'll do it in 20..
My usage isn't determined by how much I can download given my speed, no one's is. Otherwise no one would want anything more than a 1.5Mbps connection, since you can download 500GB in a month with a 1.5Mbps connection.
My consumption is determined by what I want to consume, not by what's available, I don't finish one download, realize I'm under the cap, and go and find more stuff to download.
As long as the cap meets my needs, the faster the connection the better!
And they aren't charging extra for data at the moment, they backed off that and did some public consultations.