I'm really not surprised that it sells out (the Friday/Saturday). For a bunch of reasons. For one, it's an incredible festival and a great experience. A lot of people go year after year; some people go as many years as they can. I don't know how many people that is, but I think it's a significant amount. So there's that. Then there's the number of bands on the roster. If you truly one want to see one specific band and don't care about the rest, you're not going to go to ProgPower unless that's the only place in the country that the band is going to play. So those "one band" people probably buy the ticket right away because they're desperate to see that band. If the band pulls out, they can always try to sell them. Then then you have the people in between - they don't go each year, but they're probably interested in several bands on the line up. Changes are, all of those bands aren't going to drop off, and they'll still be happy seeing some great performances.
This year was really bad for days 1-2, but sometimes it happens that way.