How would a widespread plant-based diet influence the maximum sustainable human population on earth?

Question

A meta-analysis of studies on the maximum sustainable human population on earth found the median estimate to be 7.7 billion people¹. Earth's current human population is about 7.5 billion and is forecasted to exceed 7.7 billion in 2020². Population limits are heavily influenced by lifestyle and resource usage. How would a globally widespread* plant-based diet influence the population limit for humans on earth?

_{* such that we can assume 100% of the population follows such a diet in estimates and calculations}

Answer 1

The 2013-study "Redefining agricultural yields: from tonnes to people nourished per hectare" by Cassidy et al. from the University of Minnesota investigates this question.

The authors state that

given the current mix of crop uses, growing food exclusively for direct human consumption could, in principle, increase available food calories by as much as 70%, which could feed an additional 4 billion people

The study with 175 citations (according to Google Scholar) can be found here.

Answer 2

I did a report on vegetarianism in college, and back then-- when I had access to a university library-- I found that we would increase our global food supply by at least 40%, enough to sustain another 3 billion people. I can't find my source now, and most "What would happen if everyone went vegetarian?" articles focus mainly on climate change and the economy, but they all agree that we could definitely feed more people.

This somewhat-biased article supports the notion of feeding 10+ billion worldwide. This 2016 study focused only on the U.S., but found that reducing meat consumption could allow the U.S. to feed more than twice as many people using the same farmland.