Are you saying that a candidate being opposed by someone ineligible to oppose him or her is not harmed by the illegitimacy of the candidacy?
Not until the election is finished, that is when “harm (loss of election by plaintiff) caused by a defendant empowers standing”.
Then, once past the election, it is doubtful, considering the magnitude of the case (POTUS) that any court would wish to be the one to wreck what has worked more or less smoothly for 230+ years. Courts don’t like to get involved in politics.
Law is not necessarily common logic.