Beans have a very imbalanced amino acid profile. Protein is a category, not something specific. You need enough of every amino acid. Beef already has the correct ratio of for you. Beans do not, and also have worse absorption of what amino acids they do have. So you need to eat much, much more of them to get the same nutrition you get with some lean meat, which can also often lead to weight gain due to excess carbohydrate absorption.

The only way to get a balance on a vegetable only diet involves mixing different veggies and supplements in order to try to level the ratio of amino acids and vitamins you get. It's very complicated and honestly is difficult to get right consistently even if you know what you're doing, the average person will not be able to do this right and will get sick.

If everyone is pushed onto that kind of diet most people will do it wrong, and a lot of people will get sick as is already very common in people who try veganism. I would argue you need to have significantly above average intelligence to do it right. So at the moment a healthy large population requires a lot of meat.

I didn't say "disease" i said class of diseases, there are a lot. lysine deficiency, osteoporosis, some kinds of arthritis on the amino acid end. And then there's the vitamins that beef always has, which wipes out a huge class of vitamin deficiencies... it's hard to understate how important meat is... reducing meat consumption is a very, very bad idea for public health.