And then they will depend on your API data schema...
Yes, schema evolution is hard, but databases have many tools to help here that you will either have to recreate on your APIs or live without and have a harder time. Either way, all the trouble comes from data evolution, and any schema-only change is trivial to deal with.
Data distribution is something that varies from one DB to another, they usually have very good performance that is hard to replicate on your application layer, but are very hard to setup and keep running. But the point about control of resource usage is a good one.
+100 to this
The DB can do everything can’t seem to understand this, for some reason.
Of course, merging different backends can not be done on the backend. Either you add a layer or you do it on the client.
That depends on the DB's foreign table support, or equivalent, if it has it. It's certainly not categorically impossible with a DB.
Which we have methods of evolving. I can put my API in gRPC and know exactly which changes will or won't break compatibility. Try doing that with a database.
You don't have to. Package up necessary queries into views and/or stored procedures and grant permissions only on those. Views can also shield from schema changes.
I don’t want to give other teams direct access to a DB and have them Not only take a dependency on the schema, but have the ability to run arbitrary queries that may exhaust resources in ways that impact normal operations. If I expose an API, I control the access patterns and can evolve the schema separately to suit the workload.
If other teams need a replica to perform their arbitrary queries, I’d much rather have them using a richer data model that they can normalize into whatever form suits their needs than have to conflate that into a source of truth data store.
If you have a single business unit and can get away with commingling concerns within a small team, great, throw it all in a single DB. If it makes sense to split, however, do it quick and early to avoid a decoupling hell that is more expensive then having split in the first place.