Yea but you could make arguments that whatsapp could suffer any number of ill-fated results. There's no business without risk. Technical risks are difficult but whatsapp has only shown that it can charge $1/year. $19 billion is a bet (and a risky one) that they will be able to squeeze more money out of those 700 million users.
Whatsapp was not bought only for its revenues or revenue potential, but also to protect against them turning around and using that userbase in a way that could turn into a threat to Facebook (whether themselves, or by selling to someone else).
