Firstly B12. B12 is not found only in animal foods, nor is it made by animals. B12 is made by certain bacteria (it is technically bacteria poop). Certainly vegetarians and vegans (who don't supplement) have higher rates of B12 deficiency than meat eaters. Some studies have shown as high as 90% for vegans who don't supplement. However, also for meat eaters it is as high as 40% deficiency. So actually, even meat eaters have B12 deficiency at quite a high rate, and poor absorption is one of the reasons for this. Another contributing factor to this is that generally vegans eat less processed food (hence fortified food with b12) than non-vegans. Actually nowadays vegans are more aware of B12 and soon vegans as a whole will be less B12 deficient than the general public due to this hyper awareness.

So actually the best and most abundant "natural source" of B12 is drinking water. Because everyone drinks water, this was never a problem in the past no matter what we ate. However in modern times water treatment makes this no longer a viable source of B12. Some farmers actually supplement their livestock with cobalt and B12 because they aren't consuming/producing enough themselves :p Unwashed vegetables are another source.

Aside from all that though, it's completely true that vegans (and many non-vegans) need to supplement B12. It's worth noting that most western countries have very low standards for B12 deficiency, usually around 150-200 serum levels. A country which is more onto it is Japan which set the standard of 550 minimum decades ago. This is because you can have asymptomatic B12 related ilness between 150~500 serum levels i.e. elevated homocysteine levels. So levels of deficiency are different depending on which standard you use. Anyway it's a complicated topic, but basically you win this one, "natural source" of B12 in modern times is unreliable. The caveat is that many meat eaters are B12 deficient by western standards. And by Japanese standards, most meat eaters are below the level needed to prevent optimal health and keep homo cysteine levels in balance. The amount of B12 many meat eaters are getting can be quite low due to poor absorptino, cooking (some methods destroy more than others i.e. grilling, roasting, frying, which meat tends to be)

Secondly this idea that plants aren't "complete proteins" or don't contain the all the amino acids is a myth. You can see a list of foods that contain high amounts of each essential amino acid here. Here is a short article which shows studies indicating a plant only diet supplies adequate amino acids. This article has a chart showing the amino acid profile of various foods, and you'll note all of the plant foods contain all of the amino acids (except bottlegourd has 0 for 1 amino acid for some reason). Here is a chart that shows lysine is not difficult to get from plants

Now we didn't really need all of this did we. A bit of common sense would have told us that the herbivores we eat get their protein from plants and that eating plants instead of plant eaters would be fine (if not better). Considering the only thing meat has the plants don't is B12 (and technically vitamin D but that's a non-issue), is it really worth eating it? Just for that little bit of B12 which is probably not even enough to get you over 550 serum levels?

Meat, dairy and eggs are acidic

Animal protein accelerates aging and contributes to diabetes through the mTOR enzyme

Saturated fat is pro-inflammatory

Cholestrol linked with arterial thrombosis

And... sulfur containing amino acids cysteine and methionine are very high in animal products compared to plants. Reducing methionine is an effective cancer treatment and has been documented as valid life extension strategy.

Ohhhh... and let's not forget that meat, which people love to grill , roast, and fry produces heterocyclic aromatic amines which are carcinogenic. Ohh and also the WHO reviewed over 800 studies and determined red meat is a type 2 carcinogen and processed meat is a type 1 carcinogen... I could literally keep shitting out these vegan nuggets for days. It's like a never ending list.

So I guess at the end of the day it's your life, you can do what you want. Just I think we should avoid using false apprehensions and misinformation to justify what we do.

Edit: Just one more bro... even vegan junk food is healthier than meat

:)