I understand you already can... I was just saying that rather than have the unit perish, it should automatically be made to join its home city from wherever it is when its 'time has run out'. Then again, is it necessary? At a point, the unit has been strength obsoleted anyhow and there's little further use for it but to take it back to a city.
Though I must admit it wouldn't bother me either way because I use them for the city as soon as I get them.
I'd be against this. On obsolescence sure, but not on combat death. There should be a tension between whether you risk them in combat or save them for their buildings.