It can be defended when the system is in a rather passive form of maintenance. E.g., every month a small bugfix. If it is more than that it becomes way to easy

It can be defended when the system is in a rather passive form of maintenance. E.g., every month a small bugfix. If it is more than that it becomes way to easy to fix one small thing while breaking three. Especially if the functionality is a little or a lot complex. What it does sound like, though, is that everything was refactored/covered in tests in one fell swoop. I find that questionable. I think it is much better to start out small in an untested code base. Just cover the small piece you are working on now in tests and gradually extend.