I do not think that we have to worry about long lived troops.  A level 20 troops with two traits is still nothing compared to level 20 troops from an earlier build (HPs, lightning pike, etc...)

And I have never had a level 20 troop anyway. By turn 150 I have had a level 17 troop, but that's it.

As a flavor thing, it sounds nice, but realistically, this just gives players more advantage that I don't think they really need. Unless this comes with some improvements to AI behaviors where they figure out how to treasure their units and keep them alive, I don't think it's a good idea.