One thing this suggestion ignores is the prices of dark matter which is the only reason they exist as separate items. They range from 4 gil for tier 1 to 200 gil for tier 7. It's not a huge difference, but SE would have to make a decision of which price point to set it at. Either max level players will get cheaper repairs, lower level characters will get more expensive ones, or some combination of both where they meet in the middle.