If every member of
DVC walk a reservation, would it be sustainable for MS? No, I think we can all agree.
However this is not happening. Because it's rarely necessary a part a few days of the year and only for a few categories. And because not everyone knows what it is and how to do it. If it will ever became a problem, then Disney would change the rules like it did in the past. Or probably they'll just change the website to allow changes, something that is well overdue. As the number of members grows, the break even point for investing the money into improving the system vs hiring more people to answer the phone get nearer and nearer, it's something that will be implemented anyway and will fix the problem of sustainability.
But the OP question is another: is it ethical? I don't agree about the Kant's method to say if something is ethical.
Is using a touring plan for the park unethical? Go to easywdw and read the excellent touring plans: he suggests to arrive 40 minutes before opening to be among the first people to enter the park. But what if everyone does it? Then we would need to arrive 50 minutes before opening. But then everyone would arrive 50 minuted earlier... Will we ever start lining up at park close the day before? No because the touring plan works because not everyone do it. So are people arriving 40 minutes earlier cheating others? Should they be forced to arrive at opening time? Or should they pay an extra to be allowed to line up 30+ minutes before opening? Is easywdw unethical? (well, given Josh's sense of humor someone might say so anyway).
Is walking a reservation cheating the system? Are people taking an unfair advantage against other members? I don't think so, everyone could do it, they're just playing with the rules to their advantage. Like anticipating crowds behaviour and use a touring plan.
So, a part the sustainability of a system where everyone walk a reservation (something that is not happening and we don't know if it will ever happen), IMHO walking a reservation is not unethical and I will do it if needed, until rules will allow it.