I think a 5-4 win is possible at least. I don't believe the Constitution grants Feds the right to dictate this, so I really only have certainty wrt Alito and Thomas as we know they'll do the dickhead thing, always.
I'm also pretty sure that over the course of US history, esp. in early times, the 'counting locations' did NOT have all the votes in hand ON election day. After all, it's fairly recently in US history when there was an expectation that votes would be in transit for days (and possibly weeks) after, and then it would take a similar amount of time to count them all. Fortunately, a lot of these justices have a soft spot for 'the olden days', which could help.
This current question is fundamentally the same situation insofar as the physical votes are certified as cast on time, but there was just extra time before they were physically counted. So what? Esp. when there's NO proof of even remotely significant fraud in the system.
They might also signal they'd be open to the limitation for ex-pat and military voters as those are a bit more clearly in the purview of the Executive, and then IQ47 will take it as a consolation prize in the form of another suit.2