Where I first hit this is in Oracle. Which unexpectedly locked up at a million dynamic pages/hour served out of the database, over lock contention. Ever hit a lock contention issue?
That is the scaling problem that gives no warning. It is humming along fine with reasonable load, and suddenly is falling over. Sure, you can identify the problem query if you have good monitoring, but you can't just kill one instance because there are another 100 coming along in the next second...
Having dived into the guts of those failures, and having talked with expert DBAs for multiple databases, that failure mode is endemic to databases and nobody has figured out how to catch it with monitoring. (At least that was the state of the art not many years ago.)
Yes, there are ways to tune it and to scale it out horizontally. However you never know if you will need to.
If you've got a reporting database, as opposed to a transactional one, scaling is much more straightforward to predict and handle. From your reference to a single problem query, I strongly suspect that that is what you dealt with.