I can't speak for the OP but it is well-established that there are significant genetic adaptations to the amount of meat in the diet, or a loss of genetic adaptation for metabolizing some plant staples. This is no different than the genetics that cause significant variation in the ability to metabolize legumes, lactose, alcohol, etc. Local optimizations.

There are ethnic populations that have reduced capacity to efficiently metabolize some plant-based diets due to thousands of years of selection pressure (or lack thereof). A diverse plant-based diet won't kill them, they simply lack the enzymes to have a good experience with it because for thousands of years they had little use for those genes.

It is a relatively small population globally, as it tends to coincide with regions that weren't conducive to supporting large populations thousands of years ago. The current distribution has significant overlap with the developed world though.

I have to imagine that someone with meat-adapted genetics is going to suffer quality of life issues on a purely plant-based diet. Everyone has a set of foods like that.

Might be lack of understanding of essential nutrients and associated planning. But also might not be related to that at all. The gut microbiome is impacted by your food choices, varies from person to person, and can have severe impacts on your overall health.

Nope. Some aminoacids and compounds are only present in meat. These aminoacids and compounds are the ones which boosted our brain capacity and allowed us to evolve to that point.

I eat (and like to eat) tons of veggies, yet I feel my brain capacity declines and I crave esp. meat if I don't eat it for a long time (for two weeks or whatnot). As I said, I don't need two ribeye steaks per week. My body is very good at signaling what it needs, and I prefer to listen to it.

What I eat is Mediterranean cuisine 99% of the time, and it's pretty well balanced, yet eliminating meat is not possible for me. So, my diet is not junk food peppered by meat. It's mostly veggies and legumes (beans, lentil, whatnot), peppered with meat. Yet, I need it, and this is something I tested over and over more than two decades.

On the other hand, my wife is completely opposite of me. She can go a month or so without meat. So, not every person is the same, and assuming that every human being works the same is a big mistake made by modern medicine. For example, my brain chemistry is also different and I consume B12 much more than a typical human, so I need to use B-complex supplements more.