Prometheus strikes me as one of the most amazing recent pieces of open source tech, the query language in particular (while hard to grok for beginners) was such an eye opener early in my career. It has its warts but infinite kudos to the creators.
For an example of annoying behavior in datadog:
* Zoom in on any chart with default_zero. Eventually the spaces between the points will go to zero. You have to add a rollup whenever you use default_zero to avoid this.
* In general, you have very little control over interpolation, which is a very important part of aggregating and graphing.
* exclude_null will remove any series with any null label. You can't write arbitrary filters on series labels.
* Label names and values have to be the same to match. E.g. you can't join 2 metrics where different labels have the same value.
* The GUI doesn't support a lot of the features, e.g. timeshift.
* The available functions often can't be composed. E.g. you can't min by X and then sum.
I have a ton of complaints about the GUI too (e.g. it's broken by typing too fast), but I'll save those for later.
Prometheus's query language is a sharp tool, and you can mess it up. It takes getting used to, and Grafana is a must for making charts navigable to executives (which is one of the top selling points of Datadog: Execs can see and build dashboards for the metrics they want). I would not be likely switch to another system - Even Monarch, Google's next generation monitoring system, has significant downsides compared to what you can do with Prometheus.
And the customers for this are businesses. Businesses spend money. (as opposed to what a lot of developers think)
AWS is usually super secret about the profit margin on their various cloud services, but now we know that the margin on Prometheus was at least 84%...
Amazon has a long history of being willing to eat a ton of loss to kill competition like in the very popular diapers.com business.