SHA-2 being broken is always a possibility. Keep in mind that all public crypto schemes rely on a conjecture
for which there is no compelling theoretical argument or empirical evidence,
And there are many other developments that could make bitcoins worthless, for example
* The US government bans crypto-currencies.
* Everybody chooses to use Dogecoin instead of Bitcoin
* The Bitcoin blockchain stops being maintained because
** Mining becomes unprofitable
** Other crypto-coins become more profitable to mine
** It got all messed up and the network cannot agree on the fix
and so on. Surely there are many more possible disasters no one has thought of. (One year ago crypto-currencies were believed to be untraceable and un-seizeable, for example.) That is why one cannot put a probability on "price will crash to zero"...
The nuclear industry once sponsored a very thorough safety study that examined all the ways in which a nuclear plant could possibly fail, and concluded that, assuming several hundred operating reactors, there might be one partial meltdown every century or so. That was before Three Mile Island, of course. According to the principles of that study, three separate reactors melting down down at the same time would be less likely than a meteorite killing Santa Claus; and the possibility that an unloaded reactor could explode and become a major nuclear risk was not even considered.
NASA too once sponsored a very detailed study of the Space Shuttle's safety. It concluded that the chances of a total loss were about 1 every 100,000 launches. The actual rate turned out to be 1 every 50 launches.