Simply figure out how many HBD is out there versus how many could be needed. So if you want a billion HBD, the 20% is not only sustainable, it is probably too low.
So yes, the expansion of HBD can take place at a strong rate for a long time.
Here is one article detailing how it is sustainable.
https://leofinance.io/@taskmaster4450/hbd-20-interest-is-sustainable
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Actually I read and I even commented under that article one month ago, but somehow I forgot it. So it is sustainable. Then more and more people will earn a living (or at least a very good income) with it, if they put enough money into HBD savings.