Alfred,
Airworthy is not a hard-edged condition. That's why the SofS defines airworthiness as a probability. Tuc has posted the full definition up thread. The existence of that probability is why Engines, Tuc, Chug and others all insist experience and technical knowledge is essential to make the judgement that the probability has been reduced sufficiently to meet the definition. Anyone with the competence to make the decision will want evidence as a basis.
BUT, until someone has ploughed through the paperwork trail the decision maker does no know what evidence there is to form the basis for a decision. If there is no evidence, or not enough evidence, the aircraft is not airworthy and should not be flying in peacetime. It's not just about OC or aircrew or engineers or other Service people's lives - it's about schools and hospitals and football crowds not receiving several tons of metal at high speed. It's also about integrity- being able to believe that the weapon system you are handed as safe actually is safe to go and practice with.
Retaining an aircraft in service with a known airworthiness problem can be done Sometimes, often even, the problem can be lived with, by adjustment of what the aircrew and engineers do with the aircraft so that the risk is acceptable. This has been done on many occasions, often as soon as an AAIB/RNAAIU investigation has revealed a potential problem.
Sometimes it can't . In the latter case grounding is the only option. The classic example was the Buccaneer. A spar failed. The airworthiness problem was that the spar design was not suited to the way the aircraft was being used and the ongoing test programme was based on the wrong things. The aircraft was grounded, the spars were fixed and the test programme was fixed.
N