"Course creators only create courses, they are not supposed to do anything else, the fact that they get the predefined role in new course is the only way how to make them able to actually work with the course, but it happens only if they do not have any other way to access the course."
Yes, that is correct. Typically they will not have another way to access the course at the point of creation.
"It would be imho a usability problem only if the manager did not know how to add teachers to the course."
That's a side issue. We're discussion the CC role here.
"How often do you actually create new courses?"
Potentially a lot. New site, new users, new initiative etc. TheCC role is used extensively to delegate in a safe, controlled way.
"If it was a big usability problem we could add a checkbox "Add me as a course teacher" and it would be displayed if you can manage the newly created course, but is it really worth it?"
No. Just revert to the pre-2.0 behavioiur that is understood by all and is defined in User policies.
I would personally find it very annoying if moodle make me teacher in each course I create - either as admin or manager.