The REAL Cure for Cancer

No, "harmful follow-ups" are a valid concern.  First, we can have them by guessing wrong.  We can think that the risk from radiation exposure from an extra diagnostic x-ray is less than the risk of harm from the cancer that a screening test suggests that a person may have.  If we have no clue how to interpret the results of the screening test, we should be cautious that we might guess wrong.  Second, we can be correct about the rates of false positives, and just conclude that there's enough harm.  Suppose that a screening test has a 5% false-positive rate and zero false negatives, the incidence of the disease in a certain population is one in ten thousand, the follow-up diagnostic test also has a 5% false-positive rate and zero false negatives, the treatment saves the lives of an extra 20% of patients when the disease is caught early, and the treatment kills 1% of the people who get it.  Then the screening test will generate 5% false negatives, the follow-up diagnostic test will generate one double false positive per 400 people tested, and the treatment will kill 1% of those, or one person per 40,000 tested.  Meanwhile, if the disease goes undetected, one person in 10,000 has it, and 20% of those would be saved by earlier treatment, so the disease kills one in 50,000.  The over-treatment would kill more than the delay would.  (For simplicity, in the hypothetical example, everyone is either killed or completely fine.  A real treatment would have common mild side-effects, uncommon moderate side effects, and so on.  But the same idea applies.)