Capped inventory forces you to destroy items or upgrade inventory. Also, having small amounts of left over premium currency.
"Player inventory is capped by inventory bags, 4 bags slots can be crafted, 5 bags slots are extremely rare drops from enemies, 7 bags slots are gacha only on shop (they are tradeable)"
"You can run out of space for collected moths, both in the main area and storage, it can be expanded for free but the costs get large and time consuming enough to just encourage using premium currency."
"Max heroes inventory that after a limit increases only with "VIP level". Rarely a problem though."
"ok this one is a bit of a stretch but hear me out, there are a certain amount of relics that exist in game, new ones are added every so often in LARGE amounts and there are MAAANY relics of MAAANY stars so its not like you will probably ever hit that cap but there are certain P2W only relics so theoretically if there was a maxed out P2W player they would always out compete any F2P player altho this is basically impossible with new content added almost weekly"
"Maybe 70% of the coins that the player can get from completing a level gets put in a "piggy-bank" which needs around $8 of real money to be unlocked."