While I do agree with you on some level I just don't think you can compare urban to big mountain in terms of danger. Just look at the track record: Doug Coombs, billy Poole, Shane mcconkey, c.r Johnson, Arne backstorm, Jamie Pierre....the list goes on and on. All of these guys died riding big mountains. People don't really die sliding rails. Also, check out this video and tell me it's not gnarlier than any urban crash you've seen.
Sorry for no embed, I'm on mobile. I'm not trying to take away from urban, I think it's sick, but you just can't compare if to big mountain.