* They had a massive security breach where core-db was breached. Hashed user passwords and encrypted env vars were leaked. (And a hell of a lot more I bet—but they haven't admitted this)
* They're operating with probably 1/3 the staff they were at peak due to attrition with no backfill and a layoff.
* They've been under a feature freeze for going on 5 years. Check Heroku Changelog to verify this yourself: https://devcenter.heroku.com/changelog
Yes: Heroku failed.
EDIT: just because Heroku has failed in 2022 doesn't mean it wasn't also successful. It's not too different than Flash in that respect.