I've been thinking about why a person would upgrade and what is missing from Steemit. Reddit has the erotic sections, but Steemit until this point has been relatively PG. When Steemit develops it's porn or sex related groups this is when Steem Power will become something people will have to buy.
Sex is what will drive demand for Steem Power. I do not think anything else will drive demand for Steem Power. I also would say there has to be a way to buy Steem Power with a credit card or through PayPal. Reddit allows it, and I don't know why Steemit doesn't do the processing for a small fee.