Once you give out the medals for the races this year and the challenges, I don't think you can go back and say they "don't count" from an anniversary perspective. Plus, from a tracking standpoint it would be very difficult to keep track of new anniversaries down the road. It becomes no longer simply adding 5, 10, 15, etc to the inaugural year. You have to remember and track that separately somehow and RunDisney has trouble with the simple things already...
In my mind, once a race has been scheduled, sold, set up, etc, it should count from an anniversary standpoint. It was not run due to inclement weather, but you've still got all those "20th" anniversary medals out there on the street. You can't have two sets of "20th" anniversary medals. Otherwise, it just gets to be too complicated. For example, should Wine & Dine now be offset a year on an anniversary basis because its "half of a half" was technically not a running of the half marathon? Throw in another shortened race or a cancellation in a subsequent year with this precedent and it gets really hairy to track. I say keep it simple and roll on.