Most jobs have skills that they retain at level 80 that lost their usefulness long before that. Scathe is just another of those that while you've obtained better tools, there's no real skill that can outright replace it and have it still make sense in the same way as something like upgrading Thunder to Thunder 3.
Other examples are things such as Fists of Earth becomes entirely useless on monk once you obtain Fists of Fire. Tranpose in your own kit loses a lot of its value once you have Fire 3 and Blizzard 3 and then looses it entirely once you have Umbral Soul. Ninja has no use these days for Fuma Shuriken, Summoner has very little use for Ruin 2.
These skills fill a gap that you might need/want in your kit at lower levels, that's all they're there for. Maybe they'll be culled in a future expansion, otherwise you can just not place them on your bar once they no longer serve a purpose and forget they exist.