It didn't stop attackers from using extremely common ways to punk the systems even under the best circumstances for the systems. A forgotten password gets leaked, using the backup applications/storage system's own encryption schemes against the victims, just deleting entire volumes or compromising the OS on the systems, the list goes on.
I wouldn't consider append-only an anti-ransomware technique, it just stops one of many common ways of compromising data. This is good, but I wouldn't rely on it to protect against even a run of the mill ransomware scheme.
To utterly destroy an organisation you don't erase or encrypt their data. You change it. Slowly. A little by a little. A birthday here, a name there, a number ... Using the normal ways to change this data. In this way you can go undiscovered for years, employees get blamed for making stupid errors for a LONG time and there is absolutely no way to fix things, no matter what the backup strategy is.