A permanently purchased item in the game becomes less valuable over time.
"Although this game does have ways of feeding items into items purchased to make them more powerful and thus you wouldn't need to buy a new item to stay competitive."
"Old (premium) tanks are outclassed after some years. They aren't bad then by any means but can't quite compete with newer tanks."
"They just introduced a new character level. Third new character rank."
"You can buy low-tier items, but they get useless after some time because you can get better ones"