Think the obvious solution to that would be to make the fee bigger as you upgrade more like 100 trias to start one. You do have limitless money storage (they love your money) but you can only withdraw money so many times and only three items (they don\'t make money of that it\'s service) Then 500 to upgrade then 2000 to upgrade again and 10000 to upgrade to the highest level.
Anyhow don\'t think banks are a must for players seeing there might be things like houses but for guilds it is a must to get people to contribute to their guild account and to be able to have guilds hire NPCs for work like guarding and stuff.