I would think there are far more light users who don't even come close to using 5GB than there are heavy users. For that reason I don't think we'll see metered service from the carriers, at least not to any degree that actually benefits light users. They'll probably just opt to kick out chronic data hogs instead.
A couple points ...
I think the data does indicate that the AVERAGE user uses much less data than the heavy users do so that with flat fee plans the light and average users must subsidize the heavy users, or, turning things around, the heavy users are riding on the backs of the average users. If the cell companies were to change to a tiered or metered plan and the change was revenue neutral, the average user would see a small decrease in monthly charges but the heavy users would see a big increase.
Going forward, the cell companies are going to need to control heavy use or there network will be unusable and people will leave. I can see at some point a flat connection fee for full service of, say, $30/month and then billed at, say, $1.5/GB for all voice and data. However I think we'll see tiered billing before we see fully metered billing.
Brian