Using Ark will inevitably require ongoing costs to maintain a balance. But if your app is managing refreshes properly (refreshing near expiry), costs should be totally manageable. We’ll have some projections out on this soon.
Ark is intended for “spending” money, and any long-term savings should still be held on-chain. If Ark were ever to become popular for savings, then you’d probably see the emergence of Ark servers offering much longer expiry times to cater to that (less refreshes required, but more expensive to spend).