Here are four reasons for lowering the HBD interest rate: https://inleo.io/@godfish/4-reasons-for-lowering-the-hbd-savings-interest-rate
It's one of the most commented Inleo articles. I believe the rant on historical votes to "grandfather witnesses" is actually one of the consequences of the interest rate - people have very little incentive to stake HP and actively use it (for witness votes among other things) than to vest HBD and just grab the interest every now and then.