Mortal Enemies

I was just watching some youtube and someone said 'mortal enemy'

Can I just point out how stupid that saying is, I know that I don't have any immortal enemies. I suppose that it might have made more sense in medieval times when peoples' enemies actually ended up dead but now its just stupid.

Maybe we should replace 'mortal enemy' with

'I wish they were no longer mortal because they were dead enemy'

That would be better