A court would not have to define natural born citizen or weigh the factual evidence regarding Cruz’s qualifications to determine that he should be permitted to appear on the ballot so that voters and their electors can determine his eligibility.
Suppose someone who was 30 years old ran. You don’t think anyone would challenge his eligibility? Or that the courts would rule he’s eligible if the voters want him?
It is perfectly reasonable to sue to prevent someone ineligible from being listed on a ballot. You will notice that courts HAVE ruled on Obama & McCain...