From a neutral's perspective, I largely agree that it makes sense to wait.
The problem with Zlatan isn't just his poor conversion rate this season but how he affects everybody else on the pitch. Zlatan has taken 63 shots this season in league games, which is almost as much as Martial (10), Rashford (16), Rooney (18), and Mata (21) combined. When Zlatan is on the pitch, the game will always be structured around him, either letting him drop deep to get on the ball or, more likely, pumping balls into the box for him. His movement patterns and playing style make it more difficult for other players to get involved offensively, especially from the wings, because he clogs the middle - his movement is up and down but rarely lateral - and because he demands the ball and young players will defer to a legend like him, especially one that seems to have a special relationship with the manager. I don't think it is a coincidence that Martial and Rashford have regressed with his arrival: They see less of the ball and their ability to move decisively into dangerous positions is compromised by the fact that Zlatan is already loitering in them waiting for service, accompanied by several defenders. If I had to make a call right now, I'd say that you would be better off letting Zlatan go after this year and then going all out in the summer to sign Griezmann (or, failing that, another forward who can play across the front) and building the front line around Martial/Rashford/NewSigning. But the most important thing here is that you don't have to decide now so there is nothing lost by waiting. The team is currently caught between trying to maximize performance in the short term (which signing Zlatan was clearly intended to do) and building a team for the long term (which continuing to have the attack revolve around Zlatan will impede IMO). So far, the short term is not working out that well so the most sensible thing would be to at least wait on the option and reevaluate in the spring.