If it went to SCOTUS then I have little doubt the decision would be overturned. This is also the kind of ruling that will be counterproductive to your aims since now other states are likely to move back their voter registration deadlines. Nothing in the CRA of 1964, Motor Voter, or any other law stipulates how far from the election a state can set the deadline for voter registration. So to account for future lawsuits where the deadline gets moved back x days for the next (fill in reason here) it will be moved up from a base of 45 or 60 days before the election.
This is a preposterous position to be polite about it. (And clearly ignores how the SCOTUS would realistically respond to this issue.)
Your logic does not even make remote sense. First of all, the actual specific most glaring problem with Governor Scott's decision is he ordered
mandatory evacuations and then made no allowance for adjusting the voting deadlines. Whatever you think about it, people who do to a busy schedule (or whom had just moved and had prioritized a couple other things first) may have intentionally planned to submit their voting registration requests close to the deadline but clearly in time, and suddenly found they could not do so. Someone who put their registration request in the mail with what should ordinarily be plenty of time, could easily have found that it was not enough after having bad luck with general mail delays along with specifically post offices and other parts of the USPS temporarily shut down in the area due to the hurricane further delaying delivery. In other words, part of the problem had nothing to do with the general date, but specifically the failure to make a reasonable accommodation for a specific emergency which could prevent people who otherwise would have successfully registered to vote from doing so.
The rest of your argument ignores that no state in the US has more than a true 30 day ahead of the election registration deadline, and any attempt to measurably change this to a higher number would be extremely unlikely to hold up to legal scrutiny. (The only arguable slight variation is Nevada which requires the mail in registration requests to be merely postmarked by 31 days before the election, but allows in person and online voter registration closer to the election.) The reality as noted in this thread is with computers and other forms of electronic transmission today, an argument that the logistical process suddenly requires a measurably longer deadline is not going to be an argument courts will buy. The hypothetical change is virtually certain to be struck down by the remaining provisions in the Voting Rights Act since its going to be viewed as a clearly unnecessary measure realistically aimed and primarily suppressing the votes of certain groups. The move might also run afoul of other Federal laws given the degree to which is could suppress the vote of people who have recently moved to become permanent residents of the state in question for example.
In other words, the possible consequence you're talking about it are not even a true realistic threat.