the only possible big threat are the one created by the miners with their dirty move to go with creating their own alt, or doing 51% if they don't like the current segwit activation etc...
this is more dangerous than anything else, other things, can't really stop bitcoin, like government, altcoin taking over bitcoin etc...price also can't fall to 1 usd like many newbie believe, there is no way this will happen
everyone who invested above certain threshold will not sell his coin at loss, just think about that and you should not worry anymore about the value
Cracking Bitcoin private keys are not that easy. They are very lengthy and cracking them will take infinite number of attempts by a bot. In order to prevent this from occurring, we can fork the chain, and make the wallet temporarily (may be for 1 hour or one day) de-active after 1 million or 10 million failed attempts.
not only that but exchange can stop any serious big amount of money temporarely, so you can't sell easily your coin, also with the current power of computer you can't crask the private key, in any way, not even with quantum computer actually