It costs a lot of time and money simply to keep a mission operating, and these are scarce resources that need to be used for every active mission, old or new. M

It costs a lot of time and money simply to keep a mission operating, and these are scarce resources that need to be used for every active mission, old or new. Missions are thus planned for very specific lifetimes. On an organizational level, missions that live too long past their due date cause a lot of problems, too. This isn't cynicism- it's just the reality of trying to absolutely maximize the science return per scarce dollar spent. I personally would love it if ultimate lifespan was a feasible goal for every mission. So would pretty much everyone! But it isn't.