Yeah, I was in no way insinuating that Nagios is superior in general, or even to Prometheus, just that it does the job well for some use cases. Monitoring is tricky and you definitely need a tool box because each problem has a different optimal solution.
Nagios and its forks for sure have a place in the monitoring ecosystem. They’re just not tools that tend to stick around once you’re big enough to have a dedicated DevOps or SRE team.
That depends completely on what type of business you're running. Anyone that has an environment that is unlikely to massively change (expansion excluded) within a few years benefits from the stability of Nagios. I know many large companies that use it, or derivatives of it, and even many state/military organizations.
