I don't see why decent ASICs will ever be for sale. It'll be more profitable to run them than to sell them so the manufacturers will keep them.
In practice, miners might even be a little generous in their estimates (because they are self-selecting).
And more importantly, the risk is reduced. If BTC tanks, the manufacturer will have already cashed out. If BTC goes up, the manufacture will have lost some potential profits, but will still have made a decent amount.