To play devil's advocate, you could drop the interest rate on HBD as low as 12% and still exceed what you could get from hive interest+curation and provide a greater return than most traditional investments (on average).
Having said that, I don't see an immediate need to reduce interest. I guess a better discussion might be under what conditions should a reduction be considered?
Indeed, the ~9% premium over the historical ~11% return on stocks is intended to attract stake that would otherwise be invested elsewhere. I prefer to allow the current offer to continue to demonstrate reliable returns, because prudent management of funds will await historical demonstration of stability and security of the vehicle. It would be rash to now raise that rate, despite upside room to as much as 25% or more, and I see no reason other than FUD to lower it, presently. Slow and steady wins the race to financial security, and extreme hype would attract aggressive speculation, while reduction to marginally improved returns would not overcome FUD regarding Hive. 20% feels like a sweet spot to me, and I expect it came of substantial consideration prior to it's implementation.