Good point, the reward pool has a fixed amount so if the demand increases exponentially with the user base, the price will inevitably increase as well. More than a year ago you could earn something like 100 SP for a good post, back then it was worth maybe 10 dollars; nowadays you get 5/10 SP which is worth the same amount in dollars valuation anyway.
Going on it will be harder and harder to accumulate SP through blogging for sure.