You introduce a lot of complexity when you change the boundaries. Straight to microservices is almost always premature optimization. You'll know pretty quick when it is needed.
The boundary complexity is always present. When you want to tackle that problem is another concern. Horizontal dependencies can destroy any system no matter if it within a database or between services.
A lot of monoliths reach end-of-life because everything is glued together. I even worked with partners that have systems where each database table is dependent on a single table in the database. It failed. A design doomed to fail from start.
