A permanently purchased item in the game becomes less valuable over time.
"This is very well controlled by the developers but new content inevitably starts creeping the power up over time"
"you can spend coins, diamonds and boosters once, the value of the item you upgrade cannot be used to the next chapter, so you will have to keep on spending if you'd like to finish a chapter as upgrades of Kitchen, decorations and boosters is necessary to pass the impassable levels"
"Many of the characters, sets and pieces of gear acquired early in the game are largely inferior to ones acquires later in the game."
"New gears with higher ranks make the old gears less valuable, and it also costs additional real money to promote a piece of gear to a higher rank while keeping all its current sub-stats."