> I've seen Datadog bills that could easily pay for 2 full time engineers.

Which means the companies are running thousands of hosts. So they definitely need both datadog and a full team of sysadmin/devops/SRE to handle that infrastructure.

On the contrary, the bigger you get, the more you need Datadog to scale. Once you grow big enough, you can make volume deals with them, you don't pay list price. I'd much rather pay them than have a single engineer have to spend any time managing infrastructure that isn't core to our product, especially since Datadog will always do it better.

I respectfully strongly disagree. At a certain scale when you can afford to have a full-time engineer working on it, open source tooling will give you a better, cheaper and more flexible solution since it's easy to customize. Datadog is good for the mainstream cases but it's not all that flexible for unusual or edge cases.

We migrated customers off DataDog for this exact reason.

Rolling a custom Prometheus / Grafana / Alertmanager setup is not hard at all, more powerful, and it's much easier to do it right from the start.