Every microservices deck draws the same box: one database per service, labeled as a security boundary. That label is the assumption most teams copy. Database per Service pays off for a different reason.
The coupling you feel before the outage
Picture two product teams on one Postgres instance. Team A ships a column rename Friday afternoon. Team B's release does not go out until Monday because migration ordering became a meeting, not a merge.
Nobody got paged. The deploy queue stalled. That is what shared-schema coupling looks like in week two of "we are microservices now."
For a long time one database with many services was normal. Coordinated migration windows, shared release trains, one schema everyone negotiated. The diagram said microservices. The datastore said monolith.
