Natural deaths would be too expensive and no fun. It wouldn't be cool if you saved an entire army for the end, you looked over and realized there was just lines of grave stones- they all died..
It would be neat though, is after a unit dies.. it lays there for a few seconds atleast. Slowly rots away within a few minutes. Then a little gravestone pops up thats there for a few minutes. It'd be really neat. The gravestone can be built over, but it'd be neat for looks.