One sure can have a DO droplet for a minimal rate, but unlike Heroku/AWS, they don't offer anything oriented to high availabilty, resilience, auto-scaling, performance (via workers), etc.

Exactly. In addition to what you listed, the ability to quickly roll-back to previous releases with a single click on Heroku has literally saved me from losing paying customers. I don't know what I would have done this past week where I had to do just that after a bad release.