SUMMARY:
This document shows one issue that database instance could not be deleted due to one project depending on it. This is because this database instance is set with warehouse authentication in Project Configuration. Unchecking this database instance from warehouse authentication could resolve the issue.
SYMPTOM:
"DB Instance object 'xxx'(ID:xxx) cannot be deleted because other objects depend on it" error appears when attempting to delete database instance in Strategy Developer 9.x.

DB Instance object 'xxx'(ID:xxx) cannot be deleted because other objects depend on it

ACTION:
Uncheck this database instance in Project Configuration > Database Instance > Authentication > Warehouse.