How much does one need to go after crypto currencies vulnerable to a 51% attack?
That said, the attack you describe has happened for much smaller cryptos. I'm not saying it can't happen, I'm saying there's no reason to assume it would be a huge threat to Bitcoin, because the actual risk for a user is vanishingly small. There are much bigger threats to Bitcoin's valuation that are far more plausible, such as government crackdowns.