Currently, players are putting on a bunch of low-star equipment to soak up heat for the slim chances at forge boxes when upgrading them. However, there's much more incentive to keep cycling out weak equipment than it is to commit to maxing them out or upgrading them. Why bother taking on the high costs of building up to 5* items when you could keep buying low-grade items and vendoring them instead of crafting them to 3* and above?

If players were guaranteed a star-appropriate Forge Prize Box when maxing out an item, it'd A) reward them for sticking with an item past its cheaper early level costs, where the odds of a forge box are not observably better at higher levels except at Lv9->10, and B) incentivize them to keep upgrading their equipment into real, finalized items, encouraging them to build the breadth of the game's equipment - or at least multiple variations of the equipment they like. For example, Wolver armor can end up as Skolver, Vog, or Snarbolax at 5*.

The current strategy of gimping yourself with disposable, low-power equipment for the sake of boxes could remain intact with this added to the system, since it wouldn't stop players from doing as they are currently with their weapon slots clogged up with retroactively aptly-named Punch Guns. Similarly, endgame players could kneecap themselves even harder than 0* equipment by wearing Ancient Plate - though this joke aside, that armor really should get some kind of buff too...