> If going the Jenkins route, why run it in Kubernetes when there are battle tested alternatives?
With full interoperability with Kubernetes, we can take an advantage of built-in capabilities like declarative configuration, autoscaling, auth, etc. The old static VM-based approach, might be not effective cause you need to build everything around that VM - provisioning scripts, maintenance, autoscaling, security, etc. A lot of manual work and effort at the end of the day :)
> If going the Kubernetes route, why not use a Kubernetes native CI system?
There are no Kubernetes native CI systems, Kubernetes itself doesn't provide CI/CD capabilities. It's just another software with interoperability in this environment :)
Moreover, I'd say the effort related to migration two the new "battle tested" CI/CD system might be significant. Especially, in a large, complex deployment strategies.
For people already familiar with Jenkins ways of working and willing to explore k8s ecosystem, this might be good starting point.