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.
Former employee, and you're wrong: pricing had nothing to do with it.
In 2017 we went under a feature freeze and no new features were allowed to be worked on. Anyone can look at the Heroku Changelog to verify this yourself (of course features that were in progress still rolled out over some of 2018). We had tons of attrition (without backfill) and even a layoff afterwards. Heroku is probably 1/3 of what it was at its peak around 2014.
SFDC gave up on us because we wanted to build a developer platform and they wanted us to build an enterprise platform. We sucked at enterprise features so they gave us the axe.
I appreciate you letting us know what happened exactly. This just confirms the basis of my previous comment that this is exactly SalesForce's strategy regarding Heroku: Milk it until it dies.
I cried when Periwinkle was leaked: that's when the writing was on the wall and even the most optimistic of us realized we were doomed.
I appreciate the community seems to care about our story so much.