I am far from an expert on the subject, but looking at what someone like TM4450 said for instance:
"not only is the 20% interest on HBD sustainable, it is far too low. We need to generate a great deal more HBD at a faster pace."
He is also talking about having the interest at 20% for 5, 10 and even 20 years and gives us different scenarios in this post.
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