I think Mccain does have standing, even though the election hasn’t happened yet. But damages are obviously more apparent after the election if he’s defeated.
Again, though, McCain’s house in these matters is made of glass, so he’s not gonna throw stones.
What really seems to be the reason that courts try to avoid getting involved in this stuff is that it all concerns the operations of the executive branch(separation of powers), and furthermore the judiciary appears to be insisting that the voters are in the ones in the best position to vet and not elect anyone who is ineligible.
The courts also seem to say that nobody would be disenfranchised by having an ineligible candidate on the ballot, because they can always write-in someone of their choice!
I don’t agree with such a chickenshit attitude on the part of the judges, but heck I’m just a common slug.