1. Quantum computers will crack private keys from public addresses. Obviously this means the rest of banking, the military and government, communications, and everything else are up for grabs too. However, people are working on making quantum-resistant solutions in those spaces. I'm worried that the Bitcoin developers are too few and too slow to develop a solution in time. Even if and when they do, if it requires all people holding Bitcoin to move their funds to a new wallet, that will have countless problems; just to start, lost wallets will become "known", conspiracies will abound and people won't know if they should trust the new protocol, and all lost or un-transferred funds will go to a single entity who could then control half of all Bitcoin (and what if it's China, and Bitcoin is worth 100mil USD and is the global reserve currency?). But then again, maybe it's just a normal fork.