Thanks for your feedback :)

1. I think I addressed this: they undoubtedly dogfooded a leaner product with fewer features, and that was the real MVP

2. It's safe to say that in the case of a better email client, at least 1% of the people who agree it's significantly better will pay for it (e.g. Superhuman has already proven that some significant number of people will pay), so monetizability was a smaller question mark that needed proving, compared to the question of getting people to overcome the friction to switch.

3. Ya, I'm confident they did an appropriate amount of early lean-MVP validation, and I go over some of the evidence of this in the post.