There can be no question that FODc in a fuel tank is an undesired condition. As seen in most accidents, it is not a single failure or condition that causes an aircraft to go down. It is the combination of conditions, the proverbial, when "the holes line up in the Swiss Cheese".
Tank entry is a serious business, particularly on an aircraft that has been in service. It can be a life threatening occupation and the atmosphere inside a used fuel tank is not pleasant. Strict procedure are needed to purge and render the tank safe for entry and a dependable breathing system and protective clothing are required for a wet tank. One expert I knew was Eddie Oakes who was known as a tank sealing expert back when I was working. He told me of participating in tank rescues where a sealer was wedged in a tank. They had called in the local Fire Rescue people who were preparing to used a Rescue Sawzal to actually cut the wing open to get the man out. Eddie swore that panicked people, lodged in a tank, actually swell up. He went in, calmed and talked and assisted the man out.
Getting around inside a tank with its structural members and sharp edges and tight spots is a hard task. Still, the tank sealers and Quality Control must do what is needed to at least minimize the FOD in any tank that has been entered.