Wasn't there a global food crisis in the 1970s? I think that had a part in inspiring the book, Diet For A Small Planet. The number of vegetarians may have grown in the last 40 years but I seriously doubt that a majority of people will turn vegetarian or vegan over the next few decades. It would be great if this happened, but it seems like wishful and doomsday type thinking.
There were around 4 billion humans in the '70's. In 2050, there should be 9 billion. I would think that would make a difference.
The SIWI report this is from is REALLY worth reading and I'm looking forward to the release of the book these numbers are based on: http://www.taylorandfrancis.com/books/details/9780415539692/