I wonder what the definition of "completing" the story is in those studies. I can see a whole bunch of other factors playing into that don't finish the story, like player pretty quickly decided it wasn't the game that was advertised or didn't like some mechanic of the game. Then there's the whole lower attention span of the younger generations to consider.
I appreciate that Tyron and Co. have made the game have non-story options. And even in the story option, the story is still optional. Enjoying the vibe of survival? Don't bother giving that one trader that one item and keep on cruisin'.