> The only reason I see for breaking the app this way would be to enforce low coupling.

Which is a big deal...

Microservices enable easier code re-use and division of labor across teams, especially when persistent data is involved. It is a technical solution to a people/economic problem and not the other way around. At a high level, I think of all the popular APIs powering the web (Stripe, Google Analytics, Twilio, Intercom, etc.) and how they enable developers to do their work more efficiently.

"Buy vs build" was mentioned as a promising attack on accidental complexity in "No Silver Bullet". If you design your microservices in a way that they're usable in different contexts and don't build at all when a paid or open source solution is available, you will boost your productivity.