Capped inventory forces you to destroy items or upgrade inventory. Also, having small amounts of left over premium currency.
"Almost everything can be expanded and usually requires gems to do so, of which a limited amount can be freely obtained over time."
"Buying a membership doubles the size of the player's bank from 400 stacks of items to 800 stacks of items. There are also members' items that allow for more efficiently carrying things in the inventory."
""Piggy bank" system. Your piggy bank fills slowly every stage you win, and you have to pay to crack it open to get the premium currency you've "collected" over the stages."
"Premium purchases allows to expand the inventory (temporarily or permanently)"
"Somewhat, you can buy extra storage but over months it becomes less of a problem."
"Gold Pass subscribers can earn 600 coins a day where normal players can collect 300. They can also earn more XP per day, which can level up karts, characters and gliders, which then give more points when used."
"You can only hold a certain amount of potions, ingredients and energy. You have to spend gold to increase the limits."
"Protected resources are dependent on levels of player advancement and researching. This same thing applies to ships."