I agree Matt, well said.
The folks that designed the Venturing program put a lot thought into it. I'm still impressed by how great Venturing could be. Unfortunately, because of the factors you've mentioned, we rarely see Venturing live up to its potential.
I think Venturing is the canary in the coal mine because it really relies on youth leadership and there is much less incentive for adults to fix things. What I saw of venturing was really poor leadership and teamwork skills resulting in a loss of interest. The few crews I've seen were run like GSUSA troops in that it was a few adults that held it together until they aged out and the crew collapsed. I'm sure some were/are great, but not what I saw. I'm not at all surprised by those numbers.
I think it's entirely because the youth have less opportunity to learn people skills outside of scouting. They have less opportunity to solve their own problems and are under more stress to perform. They just have further to go when they get to scouts/venturing and we, honestly, don't know how to teach it or make it appealing to the youth.
Fix venturing and the fix for scouts will be obvious.