While I understand what you are saying about the 3 extra hours... think about it this way:
Let's say it the festival is pulling in 50-60% locals (2 hour drive or less) - which I believe it probably is because there have been higher than normal numbers not just on weekends. Local kids get out of school about 3PM... if they drive to SDC, that puts then there near closing... so why even go for an hour or less? When staying open until 9 or 10, it makes going much more palatable to parents. Or for that matter... local people who work until 5 and then drive... no point when closing at 6. But it makes it a bit more worth it if oen until 9 or 10.
The big question is are the hotels and the like seeing big spikes in people staying there? Or is it truly locals? I have been going to Bucket List and the Paddlewheel at Branson Landing a lot lately and I have not seen a huge spike in the number of people there or in Branson Landing that you would expect from vacationers. I mean, yes, weekend days at Branson Landing are generally nuts, but not as high as I would expect for people specifically vacationing for this festival.
My guess is that there has been a huge bump in attendance this year, but on same days I expect it to drop next year. As I struggle to believe that people are specifically going to plan vacations just for this festival. I believe the bump has been locals... I wish I had a way to prove that, though, other than what I have seen