I like your thinking, but one minor clarification. BigQuery's actually introduced Flat Rate [0] ( a year ago) and Committed Use Discounts [1] (Amazon RIs are similar) since that's kind of what some Enterprises want. These are optional and flexible.
I personally still hold that pay-per-use pricing is the cloud native approach [2], the most cost-efficient, and the most customer-friendly. However, it's unfamiliar and hard to predict, so starting out on Flat Rate pricing as a first step makes sense.
( work at Google and was a part of the team that introduced BQ Flat Rate)