I know something is going to happen if disk space runs out; I don't need to experience it first.

Sure, but for every alert, there is exception.

ElasticSearch for example can be configured using ILM policies to fill up the disk then start deleting old records. I don't need to be woken up for disk filling up on those nodes.

Even worse is CPU/RAM alerts.

	▲	ajanuary an hour ago \| parent \| next [-]
		The number of times I've had to explain how the JVM heap works...
	▲	esafak 4 hours ago \| parent \| prev [-]
		Alerts are for when things don't go as expected. You set up log rotation but an agent quietly breaks it or ES introduces a bug in it.