I do think the interest rate on HBD is okay and for the fact that it has been sustainable till date I do believe we can only get better. At first I never knew of the benefits of saving HBD but ever since I knew I've been taking advantage of the opportunity because I do believe that in one way or another I'm helping the Hive Blockchain by doing so and I'm even benefitting in the process.
Like you mentioned, the 20% APR is doing so well and I'm very sure it will continue to do better, hopefully.
RE: Final Thoughts: 20% HBD has been a Smashing Success.