I really don't believe it's Arnold's choice.
His sole vendor these days are his most popular roles in famous franchises, otherwise it doesn't look that good for Arnold at the moment, with most of his comeback movies bombing hard, and he's very aware of this fact.
Arnold was and will always be a professional businessman, and being in another Predator will only bring benefits to him, either way you cut it.
If Shane wants Arnold, he gets Arnold. If he doesn't end up in the movie, it's because they didn't ask him.