I think when the user base grows, the amount of investors grows with it fast enough to pay for it. Only a few percent of the writers is able to break through to the top earning posts, and all the others will only earn small amounts. They (or at least a part of them) will find out they can make a quite steady income from holding SP and do some curation every day. Most will enjoy it and the earnings, when invested a bit are worth it. This means many will invest in some SP, and after seeing the result in daily income they will do additional investments. If the price of Steem goes up again, also many speculative investors will join.

I think many people will get addicted to growing their SP account and will invest regularly.