Thanks for using Jenkins for close to 10 years and sorry to see you move on, but I just want to correct the record here because I don't think the time line of events and your description are accurate.
First, pipeline stages have never been restartable in Jenkins from the beginning of Jenkins Pipeline. It wasn't as if we started with restartable stage and decided to close-source it at one point. From the very beginning, it was a feature we exclusively developed for CloudBees products.
From time to time, we do move some features from products to Jenkins. As somebody later in the thread pointed out, in JENKINS-45455 we are doing just that. Another example of this from early days is the folders feature, which is now used by many.
Any company building enterprise products on top of OSS will likely keep some features in products. And for any given person, only some of those features are likely useful. So while I understand the frustration of "that feature should be in OSS" or "I should be able to just get this one thing for a small price", I don't think there's anything inherently bad about these practices.
As for Pipeline stage view, it is still available today, and IIRC it is also still a part of Jenkins 2 default experience. Now, you are right that, as a contributor of the project, CloudBees is focused on pushing Blue Ocean forward. We think Blue Ocean solves the problem of pipeline result comprehension a lot better, and we'd rather make one solution better as opposed to work on two separate things that solves the same problem simultaneously. That is not to block other people from carrying the pipeline stage view forward, though, if anyone is willing.
I hope that helps,