Using a single DBMS for many purposes because it is so flexible and “already there” from an operations perspective is something I’ve seen over and over again. It usually goes wrong eventually with one workload/use screwing up others but maybe that’s fine and a normal part of scaling?
I think a bigger issue is the DBMS themselves getting feature after feature and becoming bloated and unfocused. Add the thing to Postgres because it is convenient! At least Postgres has a decent plugin approach. But I think more use cases might be served by standalone products than by add-ons.
It's a normal part of scaling because often bringing in the new technology introduces its own ways of causing the exact same problems. Often they're difficult to integrate into automated tests so folks mock them out, leading to issues. Or a configuration difference between prod/local introduces a problem.
Your DB on the other hand is usually a well-understood part of your system, and while scaling issues like that can cause problems, they're often fairly easy to predict- just unfortunate on timing. This means that while they'll disrupt, they're usually solved quickly, which you can't always say for additional systems.