Wouldn’t it be nice? Is this plugin territory?

Yes, this would be nice in some instances. FWIW, you are basically emulating git. If you wrote your content in a text editor (which you may want to, now that Gutenberg is looming over us all), you could do this with git, and then just copy and paste your finished piece into WordPress. Not quite the workflow you’ve described above, but close.

Do you have links to those plugins? Maybe someone will see this and think about adopting one of them. (You could also post this in Reddit, as it has a fairly active WordPress community.)

If it’s not part of core (and it won’t be—everyone is apparently busy on Gutenberg), then this is plugin territory, as it deals with behavior, rather than appearance.

Here’s a plugin which exactly doing what I’m suggested: https://www.oasisworkflow.com/documentation/working-with-workflows/revise-published-content

Oasis Workflow will allow you to revise published content and then you can submit the revised content into an editorial workflow.

Once the revised content is approved via the workflow, the new content is copied over the already published article.

As a result, your published article never goes offline for any edits.

Hope this helps.

Do let us know know if you have questions by submitting a query via – https://www.oasisworkflow.com/submit-a-query