Stake is not interest.  It is a superficial mimicry of interest..  Coins cannot earn interest until one of them comes up with a way to lend them at a contract interest rate and have them still be spendable,  but revokable (or rate adjustable) if the terms are not met.  This would never work with anonymity unless it becomes possible to enforce the contract terms on an anonymous wallet or address through some kind of escrow mechanism.  Even then there's no way to estimate default risk and thus set an interest rate to lend them at.