Calendar delete time doesn't work logically

So we'd like for old calendar events to be deleted after a certain period of time elapses following the final date(or only date if it's a one time event). But to delete an event has to recur. I don't understand why one time events can't be set to delete after a certain period.

It seems to me what would make the most sense would be to say "Delete X days/months/years after the final event date." If the event recurs it would never be deleted until such a time that it no longer recurs. If it doesn't recur it would still be deleted X days/months/years after the last date regardless of whether it's a one time event or has multiple dates.