Stereophonics still getting those bill-topping spots elsewhere - like IOW next year - pretty much means they'll go where they get the place they want. Don't think it will necessarily be a money thing - just think they don't really want to play what they see as second fiddle.
One of my favourite bands going back years, it's weird I can't see them wanting Pyramid headline as they still accept the likes of Neighbourhood Weekender and Kendal Calling. I just don't think they've got a spot at Glastonbury