It isn't the curmudgeon that was the problem; it was the lack of a credible debate, or even someone with a well-argued alternative view. That's the problem: the one-way "debate". Which is why it looked like, and was, preaching.
Oh, yes, Heinlein has quite a few characters who preach, such as Jubal Harshaw in Stranger and other books, and Lazarus Long does a lot of it as well. And not much in the way of debate, either. But I love them anyway.
I suppose I get enough debate just talking to myself. I'm usually on at least two sides of everything.