Agreed. Putting data for all your use cases in a single database is a huge deal. Having different databases for each part of the application bloats so much so easily from a cost, complexity and skill set standpoint. Also, makes it extremely hard to debug issues when you have to chart the path of data through 5 different tools.

There is the challenge of workload separation and scaling each component separately but that can be resolved by pulling out challenging workloads into their own "database" albeit on the same stack.