We started as usage-based then learned a few years in (once we understood customer usage trends - key) that customers would gladly pay ~20% MORE per year for unlimited access because the customer wanted a predictable bill. We essentially rode the same trend of pay-as-you-go mobile to unlimited plans.
Later in our history we found investors (and our accountants) liked the predictability as well. It's easier to show "up and to the right" when not dealing with a few large customers varying down in usage in one quarter (even if the business was on the upswing).
One-size doesn't fit all -- usage billing has its place for sure -- just carefully weigh your model as it's a huge lift to flip it midstream.