In your email example, the sender would be in violation of the license, not the recipient so it doesn't seem like a huge problem: if someone wants to build a better email client (for whatever metric) then the users of that client either need to make their own emails public (granted, not terribly exciting) or obtain a written permission for private use (presumably but not necessarily conditioned on paying a fee)

Yeah, communication tools are not a good fit for this experiment.

What I had in mind was mostly dev tools (e.g. slipcover, ruff, uv, ...) where

- individuals being able to freely use them on personal/unreleased code, and anyone being able to use freely them on open source projects unlocks a ton of public benefit

- large tech companies might legitimately cut their ec2 costs by 50% or more thanks to the increased efficiency

- there is not much room for SaaS or maintenance contracts to generate an income for the original author

Having a short period people working on projects they are unwilling to open source are incentivized to share a portion of their cost savings seems like it would be a low-hassle way to foster a healthy ecosystem of public benefit tooling