As a quick aside, I think it is interesting that 2010 seems to be the turning point for the most recent trend. My guess is this is the first crop of movies largely produced in the post 2008 crash so executives were being cautious. After seeing it work for a few years, they just stuck with it. Obviously just a theory but I have been racking my brain for any other events/cultural shift dating back to around that time and have come up blank.
Thanks for this analysis. I did something similar hundreds of pages ago now showing that a high percentage of recent hit movies were either already established IP and/or direct sequels. People continue to claim that "good" original movies will break the pattern, but the masses apparently have a very narrow definition of good because it just hasn't been happening.
re: 2008-2010 -- I wonder if some of this is because of the MCU. Everyone started trying to build their own franchises in response to that, and many studios found some success with that. Many started looking around for other properties with built-in name recognition that they could bring to the screen and build from there.