A permanently purchased item in the game becomes less valuable over time.
"Worst part of the game. Brand new characters and dive are boosted and powerful, but it only lasts a month. So next month you could spend hundreds of dollars on the next boosted character/dice combo to stay on top."
"New cards have better skills and slightly better stats than old cards of the same rarity."
"Newer valkyries are considerably stronger than older ones of the same rank, so much that the older ones are later given out for free."
"As new characters are released, they are generally more powerful than older characters. However, older characters are often upgraded with special weapons called manifest weapons, to keep them in line with the power creep."
"Newly introduced characters in the roster are generally overpowered and only tuned down in a later update. This encourages players to spend their currency unlocking new characters."