It is not risk free ... nothing is ... but the risks of inactivity are an order of magnitude higher.

Not if you end up DEAD! Are you really THAT dense??? You are ASSUMING that you don't have an accident while mountain biking. That's why your math is totally wrong. Get your head out of the sand. Better yet, leave it there, and end up dead. Sheesh.

I'm assuming nothing of the kind. I am simply pointing out that the risks of ending up dead prematurely are IMPROVED by taking strenuous exercise DESPITE the inherent risk of such activity. The risk of ending up dead from heart disease is orders of magnitude higher than the risk of mountain biking..