Sorry you just triggered me really bad because I've written about this like a dozen times.
Think about it this way:
The most basic way I can explain it is that the yield that HBD offers is only a threat when the debt ratio is high.
There are two websites that track the ratio:
https://hive.ausbit.dev/hbd
https://www.hbdstats.com/
So right now we are at 5%.
This means that even though we are offering 15% on HBD... that's not a lot of Hive.
In fact it's 5% of 15%, which is 0.05 x 0.15 = 0.75%
So HBD yield is creating less than 1% inflation on Hive main chain at the moment.
This assumes that 100% of this inflation is being converted directly into Hive and sold on the open market, which it isn't... we have to account for some kind of growth over time for HBD which further counterbalances this number... in fact it's likely that HBD can grow larger than 15% a year which would actually burn Hive and make it deflationary... meaning we can offer 15% and it more than pays for itself.
Then we have to consider how much USD is losing in purchasing power every year on average.
This also makes the yield we are providing worth not as much.
If the value of USD is going down something like 5% every year then all of our HBD debt is also decreasing by 5% a year... not just the HBD we are printing but all of it in circulation.
I could go on to explain why LUNA is nothing like any of this but I'll spare you the details.