I strongly doubt that jenkins-x is the successor of Jenkins. To me it looks like a tool that is able to create a ci/cd pipeline for new apps.
Jenkins X is a CI / CD platform for Kubernetes. This project is a sub-project of Jenkins. see below here
After evaluating and trying to build our current monolith, my conclusion is that jenkins-x should be used if one starts from scratch and wants to deploy a spring boot app in the cloud automatically. If that is not the case and one would like to run an existing monolith in the cloud then I suggest to use the stable/chart jenkins.