It is tough to make a compelling case that any system, considered by itself, is indisputably, unequivocally, absolutely required. They point to options that help compensate for the absence of this system. They cite ambiguity in relevant aspects of the threat. Sports analogies are popular at times like this. A football team with a strong defense and a highly accurate field-goal kicker doesn't need the leagues best running back.
Some critics work themselves into approximately the same mindset about weapon systems.
If a requirement, standing alone, cannot be demonstrated as absolute, it must not be a requirement. Nonrequirements should be canceled. That would scrub nearly all of the nation's strategic modernization programs. Other cancellation enthusiasts are eyeing the Advanced Tactical Fighter, the C-l7 airlifter, readiness levels, and force structure. Armed conflict is less predictable than football.