Understood, but that merely obfuscates the problem.
You have an election. There is an "election day" by which date you must vote, in person or by prior absentee ballots, in order for that vote to count. Federal law allows a grace period AFTER the election for military ballots. There are also laws about using the US Mail, and the "postmark" law applies, too. But those grace periods, however they are worded, CANNOT be indefinite. At some point, you MUST declare an election result because the system is geared to produce a result by thus-and-such a date in order for that result to be implemented. A late ballot can't hold up a process where an office must be filled or an infrastructure building process must begin or a law must be enacted...
So the question has to take into account that your federal grace period merely kicks the can down the road regarding the final goal of an election. The final question remains: At what point is the election process finished such that subsequently arriving ballots must be discarded?
You have a grace period by law and you have an election completion date, also by law. In the case that they don't align, you have a conflict of laws. Which one wins? If that isn't a fair question, then elections mean nothing. Government is about deciding issues. If you delay that decision, you can't do so indefinitely without thwarting the purpose of holding an election. It would be the equivalent of a filibuster as a tactic to stop something from happening - and that would mean the end of a functioning government.
Once the "grace period vs. decision date" question is decided, a voter's failure to take into account any delays inherent in his/her chosen voting process cannot be called "voter disenfranchisement" - it is either bad luck or voter negligence.
