For future New Jobs

[0blivion]

They can still add gear without having it to be specifically onto ARR EX fights, like how NIN was able to wear some striking pieces at those levels.

That is not how it works, at all. Old ARR trials, at times, gave weapons on normal mode as well, i.e. Garuda (Hard). And because SE wants a 'polished' experience - whether or not they succeed - that means putting the level threshold high enough to avoid issues regarding the ARR stuff, as well as dungeon weapons such as Lakeland weapons (very pretty and also probably obnoxious for them to do that for every leveling dungeon!).