How would you counter an author who says their heroes have to be overly-powerful Mary-Sues or they won't triumph over the villains and their villains have to also be overly-powerful Mary-Sues or their getting beaten by the heroes won't be impressive?

Well, not everything has to be super-impressive in order to win fans. I don't need to read about some overblown character that doesn't seem realistic to me. I do realize that everyone's taste is different, so what I like isn't necessarily what you enjoy reading about. I'm not into flash and bang in a book.

I want to read about believable characters and how they overcome obstacles. 

As is so often in Hollywood, how do you overcome the impossible? By creating such a fantasy-ridden storyline with poofed up characters acting in unbelievable ways, where each story has to be bigger than the previous one. What happens when you reach the top? You have nowhere to lead your reader.

In my opinion, simpler is better, but that's just me.

I wouldn't counter them.  It is their work and they are allowed to do it any way they want.  Doesn't mean that I have to buy the book, or go see the crap movie made from the book.