Hard to do...
...because you can't just "park" a spacecraft. In outer space, gravitational forces coming from all the celestial bodies tend to yank the craft, kinda like how a sea ship can end up off course due to currents. Unless it can make its way to what's known as a Lagrange Point, there's little likelihood of it being in any predictable location in the future.
And even if you could pick it up, why would you want it? It wasn't built for re-entry, so it'll probably go all to pieces once it gets back.
As for a tertiary mission, it's kinda hard to do with (a) no comets due in for a while and (b) not enough fuel in the tank for any serious work in any event. It's hit an EOL condition, so it's better to finish it off in a predictable fashion. Since there are no nearby uninhabited celestial bodies for a crash landing, the next best thing is to send it out of the way.