All the reasons you give Rev. Powell are plausible reasons for what Ebert said, but I just noted something after I posted my reponse to your post. What he is saying, in so many words, is that franchise films, 3-D event movies, and comic hero stories cannot have plausible characters. Now, of course, many of us know that is wrong. So, can we trust his opinion when it comes to these type of films?
You make a good point as well. However, I think it's safe to say that these types films are less likely to have well-developed, original characters rather than just stock characters. I think his point is that all those types of movies tend to play it safe and conservative and not take chances or explore uncomfortable realities.