I don't expect it to reduce the number of HP holders.

If I had to speculate on the most likely impact, I would guess it might encourage some HBD recipients (e.g. rewards from posts, etc) to stake the HBD for interest and keep it on chain rather than selling it as fast as possible. But that's just a guess as to a possible impact.

I think the stakeholders that are asking for this are mainly trying to create an attractive investment opportunity for cryptocurrency investors outside of the current Hive community.