What if those boundaries change because an initial assumption turns out to be wrong?
In a monolith, no biggy. With microservices, huge pain.
Moving to microservices is something you do to optimise scability, but it comes with costs. A major cost is the reduction in flexibility. Starting a new not-very-well-understood thing needs massive flexibility.
I think we've found in many situations that the microservice boundaries make it easier to change things. We were pretty careful in making fairly conservative choices initially though, and split some of the bigger systems up more over time.
In a monolith, no biggy. With microservices, huge pain.
Moving to microservices is something you do to optimise scability, but it comes with costs. A major cost is the reduction in flexibility. Starting a new not-very-well-understood thing needs massive flexibility.