Our diet is NOT subject to evolution but it's a matter of our conscious choice. This is one of the biggest misconception that our diet can evolve from non-vegetarian to pescitarian or flexitarian or lacto-ovo-vegetarian or vegan or anything else. These are all labels we have chosen to categorize human diet habits. Carnism too is a habit and not an evolutionary milestone. Omnivorous can digest meat as well as plant diet. So switching over to purely plant-based diet doesn't require any evolution but a conscious decision to abide by it. In my country millions of people are vegetarian for past thousands of years and yet some of them switch over to a non-vegetarian diet. What's evolution then!

Evolutionary reasons are just another excuse to defend our non-vegan diet. Today humanity has not left with a long time frame of 1000 years to make this switch. The circumstances demand an immediate switch. And it's not even a choice now, it has become mandatory for our species' survival. It's kind of "Do or Die" situation today.

So yes, you are right that it's not going to happen, we need to make it happen, else it's never going to happen on its own.