I've been performing and teaching and directing improv for 20+ years—for performers and suits, adults and kids, professionals and dabblers—and I totally agree that it's life-changing.
And I think I may agree with 90% being bad—if "good" means "performance-for-money worthy." But if you look at it holistically, I don't think it's a bad thing.
When you factor in the amount of time people spend in classes and workshops and rehearsals and corporate workshops vs. on stage in front of an audience...and the number of people who improvise for fun or therapeutic reasons and not to be "good" at it...and the number of new improvisers and troupes or just plain bad performers and groups...and the number of good groups having bad nights...10% is probably about right. And it's nothing to be ashamed of. If we don't fail that often, we're probably not pushing hard enough.
I'd bet the "success" rate of other art forms and artists is pretty comparable.