A window of opportunity doesn't guarantee infection. Leaving your door unlocked doesn't mean you'll get robbed; it just elevates the risk. So the question is, what is the baseline risk? At what frequency do our cells normally become become malignant growths, and what percentage of these are prevented by these genes? I don't think we're in a position to know the latter as yet, as the idea of disabling them seems relatively new. Are those genes stopping 10 tumors a week, or less than one a year? If you gave it a week to reattach a finger, what is the actual chance that, in that timeframe, a dangerous tumor would take hold that these genes could have stopped? If that risk is 10 to 1, that's a hell of a risk. If it's .01%, I say go for it. If it gets a tumor, cut it off and try again, the odds are sharply against it happening the second time.
I won't call this a terrible idea until we know the risks. Right now it's just a new idea that some people are concerned about.