We evolved as opportunists and generalists. Humans are capable of taking nutrition from plants or animals.
> and only evolved to eat plants to supplement some small things
I believe this is backwards. There are examples of hunter-gatherer societies with meat-dominant diets, but more typically hunter-gatherer diets were/are predominantly plant-based. In most environments hunting was a less reliable source of food and more opportunistic.
> But really the main benefit of meat is that you get close to the right concentrations of everything by very definition.
Not really. Maybe if you were eating chimp... but chickens have very different compositions to cows, pigs, salmon or polar bears.
> You just put a steak in your mouth and you are 80% of the way there.
It really isn't that simple. For example, it is well established that excessive red meat consumption raises the risk of colorectal cancer and other bad things.