All the on-chain data suggests that 20% is appropriate.
Simply the fact that we got through an entire bear market without the debt ratio going up is evidence enough of that. Hive (the collateral for the HBD debt) dipped over 90% from peak but our debt ratio has been in the 5%-7% range the entire time.

The main reason to support 20% is that 20% creates a flywheel that will almost certainly create more than 20% growth in demand for HBD. This means that we can basically offer 20% for free because the demand to actually hold the debt will go up by more than 20% year over year. In crypto, 20% is actually a very small growth target as coins like Bitcoin can do 70%-100% per year.

There is a very real chance that stabilizing yield to 20% creates a kind of stability and trustworthiness that doesn't actually drain the network of any value but rather adds value to it.

We can think of high APR on HBD as "going long" on Hive while low APR on HBD is "going short" and "paying back the debt" by removing incentive to hold that debt. Once Hive is over $1 I might advocate reducing the yield. Once Hive is $2+ it would be very appropriate to lower it so that we have room to increase it again later during harder times. This is the network equivalent of buying low and selling high. High APR when Hive needs to buy low and low APR when Hive needs to sell high.