We currently have two systems of getting interest.

One is from vesting. Aside from individual vest balances on user accounts it requires keeping only one additional global balance. There are no interest payments, only VESTS gain value over time automatically. No user action is required, no extra vops to track changes. Simple and efficient.

Second one is with HBD. Aside from balances it requires keeping two timestamps and a large number that accumulates balance multiplied by time it was held for - separately for each account. It requires users to send transactions in order to trigger interest payment. Such payments also need to have their own vops (extra data in account history), otherwise it would not be possible to track balance changes. Bloated nonsense.

From technical standpoint there is no contest. Vesting is much better :o)