It is unlikely (essentially impossible) that the 20% can or will last forever, but forever isn't really very interesting.

It could, plausibly, last for 10 years: If HIVE increased by 30% per year, which makes HBD 20% interest inherently sustainable, HIVE would still only be around $2 billion market cap, which is not a lot relative to other blockchains. Or even longer. Or it could end up looking excessive (and reduced by witnesses) sooner. Either way, might as well enjoy it while it lasts.