I agree. If you are in decision making position, I struggle to see why would you want to spend so much energy installing Prometheus + Grafana stack instead of just using DataDog. Even worse if your company attempts to build Prometheus/Grafana from scratch. Just why? What a huge waste of money. If it contributes to your core business then ok, I can see why you may want to build custom solutions.
Because it could be done in few commands the second time. And learning is only possible when wasting time on this kind of things, rather than trying hard to custom fit your usecase using some buyable tool.
That being said, I kind of partially agree with you and the author though. Don't maintain service just to feel that you are in control, but with the exception that if you are doing it for the learning purpose.