It's a bold move from Grosjean and it could turn out to be a very good one. He's looking to 2017 and Ferrari. Vettel will need a new team mate by then, as I am sure that 2016 will be Raikkonen's last as a Ferrari driver.
That said, I remain to be convinced that Haas will have the pace to score points. New teams to Formula 1 rarely score points in their debut season, and even though Ferrari is contributing a lot, this is still a new team, and tt's not as though they're taking over an existing one. This makes their task an awful lot harder. Ferrari engines are no instant passport to points.