He's a horrible character with a single emotion whose story was run into the ground two games ago. Replace him with literally anything and you open up the series for interesting storytelling at least. Of course, I think most people would rather SSM do anything but God of War in the first place.
How? By replacing one angry, violent guy with another angry, violent guy?
This is God of War we're talking about, now with the mythology of the vikings.
People complain that Kratos is a violent dick, but any new protagonist is probably gonna be a violent dick too. Like there's no way we're getting a protagonist THAT much better than Kratos, not with the level of violence this series is known for.