Someone in the world is going to have a use for these vaccines, buy them and then give them away if you don't need them in the end but in the meantime just book more supply. 18 months from now if the world is flushing COVID-19 vaccines down toilets due to surpluses every dollar will have been worth spending, the downsides are too great.
I doubt that is a "high probability" occurrence.
If it does occur and AZ makes a lot of money, I don't care. The EU has many shitloads of money to spend, spread out your bets, go to town, reward every vaccine producer with huge profits, that's what we want, we want all of the companies that bet big on developing products - successful or unsuccessful - to make money off this thing. That way if it happens again they won't be shy about giving it another go!