Willis has been around long enough that he will still get in good movies. Maybe not huge money makers, but he will always be one of the go to guys for people like Rodriguez and Tarantino who like to make movies for guys who like movies.
An excellent point, his career and reputation as a consistent moneymaker are already well established. Same goes for folks like Mel Gibson. But for a newcomer to the industry to espouse conservative ideals would be a career ender for sure.