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.
I personally can say that your speculation is right. I had been converting my HBD to USDT recently and moved it to other exchanges to get a stable APR. If this feature can be brought inside the blockchain itself with better interest rates, then it will be awesome and I will keep the funds in the savings inside the chain itself. 3% right now is far too low because there are other exchanges that provide huge interests.