I partially agree, but it’s just how you look at it.
If a backup solution says the backup completed successfully, but the contents of one or more source folders is skipped completely, you could be unpleasantly surprised when you have to restore files from that source folder.
Duplicati could continue the backup operation for the sources that still exist, but the main developer chose to abort the backup and throw an error message in this case. Underlying meaning is that the backup operator should regularly check and/or monitor the backups for errors or warnings. Duplicati-Monitoring and dupReport could help monitoring your backups.