As you say, the 20 or 30 GB user would pay less for using a reasonable amount of email-checking, occasional youtubing bandwidth. But that reasonable number is orders of magnitude higher than what the same user would have used 10 years ago. Innovation in improving user experiences has driven bandwidth usage way up, even for the unassuming customer. Artificially limiting that cap results in stifling innovation.
Why is that limit artificial? Unlike water or power, data transfer is not a reservable resource. Bandwidth is a shared resource, but unused data transfer capacity has zero value.
And as others have indicated in this thread, the largest providers are not on the losing end of the current service bargain. But they are losing their hold on unrelated services like broadcasting. Switching to a usage-based model from a rate-limited model in an industry where startup costs are enormous and providers make a healthy profit is strictly an anti-competitive strategy.