[saalweachter]

Basically, proof-of-work cryptocurrency isn't secured by the abstract "computation" or "work", it's secured by money. The security of a blockchain is proportional to the amount of money that is constantly poured into securing it. If this amount of money is too low - even temporarily - a malicious miner could spend a certain amount of money and expect to break the chain somehow, for profit, as a hostile government, or for the lulz.