No mater when you cancel, you then go the next five years without the option of taking a mission. Unless a mission's conditions to make it a possible mission are no longer true, that mission will never leave your list of possible missions. IE: The list does not update or change unless you actually complete the conditions of a mission or start working towards finishing it on your own.
So? Why do you need to have an active mission at every possible moment of the game. Don't like your missions and don't think they will fade away? Just pick one and cancel. That is a decent solution for the problem of having impossible missions.
Also, no mission is truly impossible. Improving relations with Morocco, for example. Just stop being their rival and, instead, set one of their rivals as one of your own. Then improve relations, give gifts etc. Sure, it is not practical, but not impossible either. If you don't want it, again, just cancel or pick something else.