It seems like collectively changing our diet is just rearranging chairs on the Titanic. Our population will continue to grow exponentially as it has for the last several million years no matter what we eat. If somehow we avoid a massive die-off as has happened with other species due to, say, rapid climate change, a meteor, or a super virus, and actually survive to the point where the acreage of food producing land has a significant effect, then our population will be so large that one or two generations of growth will overwhelm the efficiency gains we make due to diet.

Put another way, if food producing acreage is the limiting factor for humans then as omnivores we may grow for 150,000 generations, as herbivores we may make it to 150,005 generations. Either way, growth will have to stop and we will have wiped out massive numbers of other species.

If you only take into account caloric production yes, meat is not very efficient. But there is a reason humans started to eat and domesticate animals. They provide us with fur, an other byproducts (fertilizer, etc...)

I understand the need to find some substitute with 7 billion people in the planet, but i think grown-in-a-lab meat will probably be a better alternative in the near future.

When people fly airplanes across the world, do they not look out the window and see what I see? LOTS of unused land. As in, most of the planet.

There's plenty more room out there for everything we need. Only political borders (placed by us humans) prevent us from using all available land/resources to provide for all of humanity.

Exactly. Vegetarian diets are the most efficient use of natural resources.