Yeah, that's the doomsday scenario and where they need a full court, though I would suspect that particular issue would be more than a 5-4 decision in favor of no shenanigans. Illegible or no postmark at all? GTFO with that. Anyone who values election integrity should say the same.
Not only that, but the USPS went full-retard and endorsed Biden (as opposed to at least feigning a non-political stance), so who's to say they wouldn't fire up the postmark machines and have plenty available in the event of a close count.
I hope we don't see any of that.
I have Trump winning PA, though. By a couple/few points. So hopefully, none of the doomsday scenarios are a concern.
I will say same as if Biden wins PA by 3, it's over no doomsday scenario, either---absent shenanigans.

I think we'll have a clear victor.