Datadog has ~9000 customers. I imagine the respective open-source tools have many more customers than that, which says something about the perceived value of the two approaches. Also, Datadog wraps many tools into one whereas the open-source solutions have singular purposes. It's an apples to oranges comparison.
- metrics system (Prometheus, InfluxDB, Graphite, etc.)
- dashboards (Grafana)
- alerts (both the metrics system and Grafana can handle this)
If you're not a big-data company, you can self-host instances of these products reliably.
Disclosure: My startup, https://HostedMetrics.com, provides turnkey hosted versions of these software packages.
I'd be happy to explain the options that are out there and provide any advice you need. Get in touch: contact info is in my profile.