A permanently purchased item in the game becomes less valuable over time.
"some of the older characters and mechanics you can get from the gacha are much worse than others, typically because they're older"
"Pretty much everything you purchase is a token of some kind, so you use them up. Sometimes these tokens are for limited time events and the tokens expire when the event ends."
"As you progress and level monsters, the amount of food required to get to the next level increases sharply, making it very expensive to continue."
"As you level up, the costs of playing the higher levels are higher, with no discernible increase in winnings."
"horses from previous generations will eventually be removed from the game and are significantly worse than newer horse models."
"meta changes a lot in the arena, and some cookies just... become less useful as you progress."
"Things purchased with the in-game currency (which can be bought via microtransactions) only last until the end of the level. The amount of in-game currency remains between levels."