This is really up to debate.
I really don't know what would be the right approach here, but I'm not in favor of changing things all the time. We need some stability and predictability.
Is the APR sustainable or not, comes down to the price of HIVE. If the HIVE price increases the debt should not be an issue. Its similar to the countries debt in the real world. If you manage to pull out economic growth, then the debt will be negligible. We are now approaching a HBD supply crunch and I would leave it out to play out for at least a year. The HBD in savings cant grow by a lot without pushing the HIVE price.

Ultimately the haircut rule is what regulate HBD and the overall tokenomics, more than the APR for HBD in savings.