Veganism certainly has some merit (although achieving a truly balanced vegan diet is insanely complex and way too much work for the average person), but the real problem has never been feeding one person. It is an unsustainable solution for society as a whole and that's where it really breaks down. 3D printed meats are going to be a much more realistic solution IMO.
The moral argument fails spectacularly if you think about it critically for a few minutes. There is basically nothing that a person can eat that something else hasn't died for (even if you grow it yourself). If you are eating "organic rice" then you are really eating dead animals and birds from a dead river (due to big-ag runoff) in China. How many insects, rodents and rabbits do you think are killed by industrial farm equipment to get you that head of lettuce? How many fish die due to rivers being diverted for irrigation for farms (70% of all rivers diverted in some way to water crops)? How many other animals die off because someone turned their homeland into farmland? The breadbasket of the US used to be covered in Bison and wolves with topsoil nearly 12 feet thick, now that topsoil can be measured in inches in most places and it is never, ever, EVER coming back. Do we only care that the cows are being treated inhumanely because it's more obvious?
The political arguments always revolve around solving world hunger, but they never add up. The more land we put aside for these annual crops (grains etc) the quicker the topsoil is depleted without possibility of recovery. Every time you see that "1 pound of beef" argument, it generally revolves around feeding cattle grains which they are not supposed to be eating (they only eat them because we subsidize the shit out of them and they are cheap). This is aside from the fact that you can't grow the levels of crops we need without manure, and you don't get the levels of manure needed unless cows are mass-bred for consumption. Catch-22. Additionally, not a single international aid organization suggests or promotes veganism/vegetarianism as a solution to world hunger because... it simply isn't one.
Then we get to nutrition. Nutrition has been taking it from all sides lately so there is lots of new information to take in on things like salt (not as bad as anyone thought, promotes healing and heart attack risk does NOT decrease when sodium intake is lowered), fats (good for you!), grains (sugar with opioids to keep you addicted), cholesterol (does not actually cause artery buildup, excess calcium does). It's a lot of conflicting information and really requires some mass re-education to occur (which won't happen until the big-ag corporations stop making money...). First things first... some people physically can not be vegan/vegetarian because their liver doesn't produce enough cholesterol. Life is not possible without cholesterol and if you aren't ingesting it or producing enough of it... you're dead. There are tons of horrific health concerns surrounding soy and grains now. Contrary to what has been suggested below it is NOT actually easy to eat a balanced vegan diet. You have to be extremely careful to cover all of the appropriate vitamins/minerals/metals when eating vegan, and you basically HAVE to supplement certain things (B12, K2, fats, etc) in some way. It is extremely easy to screw up though, and it causes all sorts of (sometimes irreparable) damage to your body and digestive tract.
The overproduction of grains on earth has allowed for the overpopulation of humans on earth, and we are stuck in a completely unsustainable situation that no one has an easy solution to. Well, some people do... but it involves killing off about 6 billion people so we can get back to levels of farming and agriculture that won't perpetually drive the planet's resources into the ground. Farming heavy is a problem, meat heavy is a problem... the answer (as always) lies in a balance of the two.