While I can certainly understand the disappointment caused by booking a trip specifically to experience Oktoberfest only to have it cancelled, IIRC, there are all sorts of stipulations in our travel contract that say ABD (or Tauck, or
Nat Geo) can make substitutions to the itinerary.
Oktoberfest being cancelled seems sort of analogous to going on a Disney Cruise and being unable to stop at Castaway Cay. I would bet that most people book that cruise for that reason, but DCL doesn't typically cancel or give refunds for trips when the ships are unable to dock there. That's just not financially prudent. And, technically, there is MORE to a Disney Cruise than JUST Castaway Cay.
I will say, though, closures is why I opted to move our Nat Geo trip to July, even before it was cancelled. When excursions and hotels started publicly saying they were going to be closed during the dates of our trip, I decided I did not want to travel if certain things were cut from the trip. So, I rescheduled for dates when those things are open (so far).
Unfortunately, Oktoberfest is annual, so it's not that easy to just reschedule. However, I don't think expecting ABD to cancel the trip is reasonable, either. If the trip still proceeds, there will be PLENTY of other wonderful things to do besides Oktoberfest. If that was REALLY an important part of the trip for a guest, though, I would ask about rebooking for next year or on an alternate trip, with the understanding that there may be financial "penalties" for doing so. The other alternative would just be to cancel and forfeit the deposit, and I don't think this is a situation where ABD should be obligated to refund it (although, it would be a nice gesture of goodwill).
If cruises are allowed by September, I'm sure AMA and ABD will WANT to proceed with any and all trips/itineraries, even if Oktoberfest is cancelled. They really can't afford any unnecessary cancellations.