I'm not sure this is the use case for the majority of Fi users, if they are using a lot of data its going to be cheaper from one of the source carriers than Fi, there is no way Fi could be cheaper than T-Mobile or Sprint who are the Google Fi carriers for heavy data use like this.
Most of the time my phone is on wifi and only using data during travel why pay for anything Im not expecting to use. If I'm off wifi enough where I'm doing a lot of data I'm not using Fi.
> there is no way Fi could be cheaper than T-Mobile or Sprint who are the Google Fi carriers
Look at it as https://en.wikipedia.org/wiki/Price_discrimination. A carrier can charge a lot to their locked-in customers, and sell spare capacity at lower markups to people who have competing alternatives, as long as all not everyone abandons them and moves to Fi.
Most of the time my phone is on wifi and only using data during travel why pay for anything Im not expecting to use. If I'm off wifi enough where I'm doing a lot of data I'm not using Fi.