Even the poorest countries are getting wealthier, so it's time to redefine poverty?

That's the argument Bill Gates makes (WSJ):

Today, more than 70% of the world’s poorest people—those living on less than $1.90 per day—live in countries defined as middle income, according to the World Bank. Once countries cross the threshold from low-income to middle-income status, the grants and below-market loans that have helped them rise often come to an end. Countries with huge pockets of poverty like Nigeria, India, Pakistan, Ghana and Vietnam could lose as much as 40% of their development assistance in the next few years, a study sponsored by our foundation found.

For example, the average income in Nigeria is nearly twice what it is for sub-Saharan Africa as a whole. Yet, more than half of Nigerians still live in extreme poverty. And although Nigeria has a higher average income than countries like Ghana and Vietnam, World Bank data indicate it ranks lower across a range of human development indicators such as life expectancy, literacy, and maternal and child mortality.

Isn't that the point that aid is meant, not as a crutch, but as a path to sustainable development? Further, how much of the improvement in the condition of these poorest countries are the result of aid? Shouldn't that be the argument made? And why isn't it?