While sha2 is plenty secure for >99% of the world, it's still only a matter of time before it is broken. At such time, Bitcoin would take a devastating hit. One that it may never recover from, but assuming devs went to work quickly or already had a backup plan in place, it would be fairly straightforward to switch hashing algos out to sha3 or something more fitting. This then brings up another problem... Our current squadron of miners are set up to mine sha2 and sha2 ONLY. Miners will be pushed out of business immediately and new mining programs would have to be written so GPU's could fill the void if at least temporarily.

Initially, Alts would absolutely take a hit as well, but once it is realized that most of the current survivors are of a different algo (scrypt...etc) and/or method (PoS...etc) for verification, The stronger, more trusted alts would recover. There will be no capital flowing from Bitcoin to the alts since Bitcoin is now broken and no one would buy it,so very little volume would actually get executed before it fizzles out of existence. The tiny amount of capital that does make it out of Bitcoin would either move to an alt (roll the dice on which) or be removed from the system for good. Don't expect LTC or ETH or XMR or DASH to rocket to $400 overnight, It won't happen. This all assumes trust for some computer program isn't lost completely.

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You speculate for both positive and negative possibilities. This prepares you for whatever is thrown your way. You can set a plan in the (however unlikely) event that something does go awry. NEVER, not even for one second, assume that Bitcoin is immune to outside threats, NEVER! After all, it is just a computer program that was written by some guy, which uses an encryption algo that was written by some other guy at some agency. Windows, Linux and OSX are also written by some guy and none of them, even with dispensable amounts of capital, are flawless.