The 21 million doesn't mean much really. The decimal point can be moved around. You would still need to go through a middle man to exchange btc to local currency for remittances. How exactly is btc going to lift the third world out of poverty? Many btc holders have already fallen victim to theft, and many more will in the future. People of the world won't suddenly turn to btc in the face of economic collapse.
Because the unbanked of the world will now be able to save their money. As the value of each bitcoin increases dramatically over the next few years, they will finally be able to accumulate wealth, free from corrupt government influences within their countries that would desire to confiscate it.
Yes btc can be used as a vessel to save ones wealth. How is someone living on less than $2 a day who can't afford to save anything going to acquire some? The whole world can't just acquire some btc and hope to get rich. Sure some of the people in the third world who lift themselves out of poverty may invest in btc, but btc is not some magic cure for the world's ills.