When the vegans have been 100% vegan for 50 years, with absolutely no dietary supplements, and are still in perfect health, then I'll believe it's a truly sustainable, healthy diet. Actually a couple years of eating 100% vegan (with absolutely no supplementation of any kind) should be more than enough to prove a vegan diet is nutritionally inadequate, and in fact dangerous in the long run.

One thing I often see vegans and vegetarians claiming is that pound for pound (or ounce for ounce, gram for gram etc), spinach has as much protein as meat, but according to every nutrition panel I've ever seen for spinach compared to meat, spinach doesn't even come close.

This is from the nutrition facts app on my phone:

100 g raw spinach: 2.2 g protein
100 g raw 70% lean ground beef:14.35 g protein

I purposely compared their ultimate spinach to non-organic, relatively high fat ground beef, because I knew the lower fat ground beef, roasts, and steaks would have even more protein. So even comparing spinach to low quality, higher fat ground beef, you still get more protein from the ground beef.

I don't know where vegans get their nutrition information, but I don't believe they've bothered to actually research nutrition stats, and are instead just following whatever their gurus tell them.