Drivechains could be sustainable. While bitcoin is rules without rulers, once you add a custodial it's rulers with rules. The rulers in this case, who choose the rules are the drive chain operators and the miners.

The miners perform a service for escrow, for which global rates are currently 1%-2%. Either this is paid on deposit, or paid on withdrawal. And should not be more than 10%. This should be in the T&C of the custodians.

If so, the chain can be in theory game theoretically sustainable, by producing enough earnings for the chain to survive. And also encourges the chain to have features so good that the depositor will want to take the extra custodial risk, and pay a fee.