We started resin.io with a microservices architecture from day one, and we are still happy with the result. It was very painful to get it up and running, but once that was over, we were good to go. The boundaries we defined early on are still solid, and the result works well. One critical detail however, is that all our persistent state lives in one place, minus specific well-understood exceptions. Arguably, starting with microservices helped us define strong boundaries we weren't tempted to blur over time.

All this said, I do sometimes wish we had started with a monolith, if only because we paid the microservices tax in deployment and infrastructure maintenance way too early, long before we had the scale to warrant it. I feel starting with a monolith would have probably meant more progress in less time, though with a risk of not being able to refactor smoothly when the time came.

Overall a hard call to make, since I'm happy with the result, but wonder about the pain it took to get here, and at the same time counterfactual universes are hard to instantiate...