I'm not so sure this is obvious. How much damage can one intelligent, rational, and extremely devoted person do? Certainly there are a few people in positions that obviously allow them to wipe out large swaths of humanity. Of course, getting to those positions isn't easy (yet still feasible given an early enough start!).. But I've thought about this for maybe two minutes, how many nonobvious ways would there be for someone willing to put in decades?

The usual way to rule them out without actually putting in the decades is by taking outside view and pointing at all the failures. But nobody even seems to have seriously tried. If they had, we'd have at least seen partial successes.