let's say we're 20 years in the future and the world has adopted bitcoin as a global currency. We develop very sophisticated processing technology, and a bunch of nerds hack all the secret keys of the bitcoin network. That is a bad future.
If sha256 is broken it'll have to hardfork to a better algo. It's working for now.
When it stops working you'll notice from the monster-dump on the market.
So now that i think about it: in case Sha is broken it will be too late to fork already because markets will be crashed down before people become aware.