In safe mode computer starts up but very slow, what I did is download the update from the downloads page Apple.com site. So redid the update but not through the App store because there it thought I already downloaded it. This resolved it. However this cost me considerable time to do and downtime from my work. This just happens to often in the last 4 years, deeply disappointing and no compensation of course for the damages I and many others incur etc. This just should not happen, never.
Sierra is still available via the App Store. I would download it and save it somewhere safe and make a bootable USB drive in the event you ever have an issue. I have always made one in the event of a catastrophe like you had. We are at the mercy of other human beings that make errors as we all do. Sorry for your disappointment in Apple. I have experienced similar issues but have always had the peace of mind that I am on the BEST OS and Devices available. I always backup to an external SSD drive before applying an update.This insures me against a "flawed" update or install. Disable auto update in your preferences. This way you can make the decision to update or not.
BTW you can always re-download apps you previously got via the App Store. Simply go to Purchased in the menu and it will be there. If not, search for it in the Purchased selection.
Get an external backup drive. Make it bootable via this app.
Mac Backup Software | Carbon Copy Cloner | Bombich Software
If you crash or experience a software issue you can boot from that drive and not lose down time and apply your backup to the internal drive.
Use this app to make a bootable USB Drive on an 8 GB or higher.