I'd say that's entirely unrealistic. Florentino would loathe to let him go after making him the most ambitious signing of the Galácticos era and subsequently elevating him to the manager's position (which was a massive vote of confidence). If Zidane does leave Madrid in the future for some inexplicable reason, he'll probably manage the French national team — seems like a very organic and obvious course of action: they will likely start peaking by the 2022 World Cup based on the profile of the players, so he'll be ideally positioned to lead the charge. Winning the World Cup as a player and as a manager would be a veeeeery enticing proposition for him, especially considering only 2 humans ever (Zagallo and Beckenbauer) have managed to do that. That will definitely cement his name as one of the foremost figures in football history.