If everyone ran off every time there was a problem (which they don't, fortunately), then everyone would have to convert all their earnings to a new currency every few weeks and potentially lose all of their money in the process.
And there's nothing wrong with that. First of all, it wouldn't be "every few weeks", second, as the "first adopters" are essentially paid by the "last losers" of a coin, there's nothing wrong with being sometimes "first adopter" and sometimes "last loser". After all, the idea of a currency is not to "hold your wealth", but as something you acquire to do commercial interaction with (selling and buying services and stuff).