> I find that the microservice first approach makes me consider future optimizations, such as caching policies, whereas, in a monolith, I would proceed in a naive, "I'll figure it out later" approach.

This goes to his point, the time spent considering these points is time gained in delivering an MVP to market.

> "Hey, we just built this great MVP for you. It probably won't handle significant load, so we're going to go off in a corner and make it do that now. Oh yeah, we won't have time to develop new features because we'll be too busy migrating tests and writing the ones we didn't write in the beginning."

You don't need that level of scaling potential in an MVP, and you get to have something out -- in time. While if you had started with a microservices approach you would still be off in that corner trying to get something usable out. Now you have a product in the hands of users, and your refactoring can consider that feedback.