Brandur I think you left just before we started to have real headcount problems which I blame for our demise. I agree with pretty much everything you said but I think the technical and product problems you bring up were actually symptoms, not causes.
A few more people and we could've properly maintained the stacks. We should've had an entire team if not department dedicated to thinking about Docker. If I recall it was a part-time concern for like 2 of the language engineers (who had a ton of work on their plates).
Heroku should not have stopped at Cedar: it should've been the one to build something akin to k8s but with good DevEx.
As far as why we had headcount problems: it's because sfdc gave up on us. See Craig[0] and Adam[1]'s comments for more on that. We certainly did though: sfdc would have entire teams dedicated to similar roles we would have individuals handling part-time. Salesforce folks would tell you we were cocky assholes and they were right—but we also worked damn hard.