A 2005 Lancet paper [1] says that ~35% of cancer deaths can be attributable to a modifiable risk factor, and that's only risk factors likely to be causal, not known to be causal. If accurate, that would would support the "most cancer due to luck." However, this is the first that I've looked into it, and if it's higher than 50% I would be quite interested to see data supporting that.

[1] Abstract: http://www.thelancet.com/journals/lancet/article/PIIS0140-67... PDF: http://www.thelancet.com/pdfs/journals/lancet/PIIS0140-6736(...