I understand why you wrote it, and I agree with you for the most part.
My only point (and I was trying to be both slightly comical yet serious at the same time) was that the last post might come from the person who solved the problem, even after the OP acknowledged the help.
If I solve your problem and you post a Thank You to let me know that the problem was solved, by your rule I'm not allowed to say "You're Welcome. Glad I could help."
That would make my response the last response which would mean that your "OP should post last" rule was not followed, even though the OP did in fact that us know that the problem was solved.