It's understandable he'd turn down a bit part in a movie for a role that may or may not develop in the future. I get that they'd be setting him up for more roles, but even the film doing well is no guarantee that the story would develop how the script suggests. Not entirely sure Noomi Rapace thought she'd be relegated to a bit part in the Prometheus sequel, and Sanaa Lathan said she was contracted for more AvP films in the commentary. Both films did well (Yes, AvP was a financially successful film!) and both developed veeeerry differently to expectations.
Arnie's old. He's achieved everything he's set out to. He really doesn't need to prove himself any more and if he feels the role wasn't worth his time, more power to 'im.