Posting rewards is one of the main characteristics of this blockchain attracting content creators, something that makes Hive stand out, and still there is not many solid, attractive apps built on top of Hive. Lowering post rewards right now seems like a dead end to me. However taking into account long-time perspective it seems to be a good idea, we lack any numbers to support it now.
As for lowering HBD APR it seems to be secondary issue as we didn't see massive outflow from HP to savings, it seems rather there is more assets coming from the outside of ecosystem. 20% only seems scandalous ;)