Healthier is debatable. Meat is, relatively speaking, pretty good from a health perspective.
Most of what we eat that’s “bad for us” is refined carbohydrates. Sugar, fried starches, breads, that kinda shit. The burger patty is far from the worst offender on the plate.
If suddenly everyone is slaughtering their own animals, the foods they turn to to replace this calories aren’t going to be leafy greens, they’re going to be shitty carbs. Shitty carbs are already most of people’s diets.