OP mentioned (and I didn't check) that 2/3 of incidents are non-controllable, and not that we can't control 2/3 of causes, that's completely different. I'm going to naïvely assume that it's much harder to have a clean slate (no meat, no pollution etc., healthy lifestyle etc.) than to live in carcinogenic to some degree environment.

As a thought exercise I'm assuming (completely sucked out of the finger) numbers in which 95% of population lives in carcinogenic environment and 5% has a clean slate. 2/3 cases being uncontrolled in such heavily swayed population would pretty much make any control pointless. If I remember correctly there's a method for calculating this, but I long forgot it. For comparison regarding chance perception "casino always wins" relates to a 1-5% house edge on games, and here we're talking about potential 33.3%.

Current knowledge is that human body consist of dozens trillions of cells which regenerate on variable rate (some couple days, some couple years). There's quite a lot of times where chance can go sideways and spawn something you don't want.

Those numbers are completely offtopic and without any base whatsoever (and once again, I didn't verify OPs claims). Spiraled on numbers :)

So yes, if you stop smoking you can have a significant impact on your chances of getting cancer.