One can get more interest now on HBD than staking HIVE

That's not a useful comparison. HIVE can go up 1000%, and has in the past. That's 50 years of 20% (non-compounded). Meaning, if you think HIVE will do well, you are better off with either HIVE or HP than HBD, and saying screw the higher yield.

There are plenty of cryptos (and other assets for that matter) which have done well without any yield at all.

The yield on staking HIVE is only useful in comparing between staked and unstaked HIVE.

Also, literally nobody as far as I know has ever said that 20% is sustainable indefinitely. It's a promotion to attract activity, and if demand is too high it'll be reduced (if not sooner). That's what didn't happen on Terra. Even as UST exceeded LUNA in market cap (debt ratio >50% in Hive terms), the interest was not significantly reduced, and no measures were put in place or kept in place to prevent or address that kind of gross imbalance.