You should know me better than that. There are plenty of production memos where they discuss discarding guest-of-the-week characters in favor of the regulars.
But they still also did episodes where guest stars played central dramatic roles. Again, the only part I object to is the overstatement that it "had to" be a regular, as if there were no alternative. If you know me at all, you should know I don't believe in binary thinking. Just because there are arguments in favor of using a regular doesn't mean it "has to" be done in every single case.