That previous pic is of Eyal from five years ago; he is indeed descending along a trail, albeit carefully. These days I tend to 29er that sort of terrain.
I would point to this post by Kris about crank length. The backstory for the trip is that we forgot to bring his muni on the second day in Squamish, so he was borrowing Eyal’s for large sections.
That said, muni setup is definitely all about what works best for you and your trails. I tried short cranks for a year or two once upon a time and unlike a lot of muni folk, I did not enjoy it all that much. I found that pedal strikes were easily averted by taking the high line. 
A rim brake never was and never will be an option (true rims are for riders with technique :p), but my knees might someday be convinced to try a disc brake on the 29er.
Bringing my foray in this thread full circle: I’m clearly in the minority with respect to crank length; rugged long cranks (above 165) are difficult to find! 