"Should we start eating vegan?"
First off your question is very poorly worded, in that it lacks clarity. There are two ways in which the question can be taken. The first is at the individual level, and the second at the overall populace level. At the individual level it's a personal choice. At a global population level it would require everyone agreeing to it, or it would require being made law that everyone has to follow the strict vegetarian diet. However the strict vegetarian diet would prove detrimental to the health of many, due to what is known as failure to thrive.
Even registered dietitians who are vegan, have admitted that people can and do for unknown reasons suffer from failure to thrive on the strict vegetarian diet, as followed by vegans. They also believe that they can fix whatever the problem is, because they think that they know more about diet and nutrition than even those who are vegans and are also registered dietitians. Now while the strict vegetarian diet may have some health benefits, those benefits can be offset by causing or even aggravating other health issues. I've stated many times in the past, there's no such thing as a one size fits all diet.
Also the claim that if everyone became a vegan, it would end world hunger. That's idealistic, and it's also actually not realistic. There's a lot more factors involved than is taken into consideration. War and politics are two of the more common causes in the modern world for starvation. Also there are droughts, that will cause people in other countries to be starving. While the answer may seem simple, the solution isn't nearly as simple as some think it is. Idealism is a wonderful; thing however it has to be properly tempered with a solid grounding in reality.