No offense, but if they were immortal they probably wouldn't have to worry about sending the elements of harmony and the rainbow power to do things. They're probably losing their power so that's why they send Twilight and the girls. At least that's how I always saw it, anyway.
Well at least they didn't turn into monsters this time. I remember when Celestia put a curse on Luna's favorite sex toy and turned her into Nightmare Moon. Still, i suppose i should resurrect them. Equestria isn't going to destroy itself, it needs horrifically incompetent princesses to help it on the way, like the razor blades to slit your wrists with.
Fortunately, they are immortal and will be fine momentarily. A cash prize is also given upon killing one or both of them. It's perfectly legal, as proclaimed by Princess Celestia herself 445 years after Luna's banishment when a heavy steel bookcase (steel was trendy at the time) loaded with books fell on her by accident, killing her for 5 minutes. Once she revived, she took care of the bookcase herself. Lighter wooden bookcases have been used ever since. Coincidentally, zombies entered the horror stage shortly after that event.