There's no guarantee, but Satoshi's paper addresses the dynamics of this - rational miners shouldn't want to undermine the validity of their own wealth. Doing things that significantly reduce the utility of the system is self-defeating even over the medium term because it'd lead people to just give up on the system in disgust and sell their coins, driving down the price.
i believe this is an analytical error that i've tried to correct many times before. for one thing, it ignores dynamic market effects (for example, someone who profits from a put option, a short sale, or even a regular sale). it is usually a mistake to predict too confidently what 'rational' parties will do based on an incomplete understanding of their behaviour.