Depending on their definition of humane, nothing. To some vegetarians, they don't really care about what the healthiest way to eat is if it comes at the cost of killing animals. So the argument of "oh well it's the healthiest way" doesn't even effect the argument.
I'm sure we could produce more meat, raised in more sustainable ways, but that would involve people taking a major decrease in the amount of meat they eat. Modern levels of meat intake are at their highest levels in history. We can easily afford to lower are intake.
I'm also not a vegetarian, just know quite a few and have been around when the arguments come up.
I've been a vegetarian (and vegan). Now I considered myself a reducetarian.
There are 3 motivations why people follow such a diet or lifestyle: environment, animal welfare, and health. It is important to realise the interests of these motivations aren't always aligned.
As an example, consider the difference of impact with regards to eating less cow or eating less chicken. If we'd eat less cow, the Co2 footprint would be lower compared to chicken due to required more land and more methane gas from cows however it'd be good for animal welfare since less cows suffer since one cow provides far more meat than a chicken. Conversely, the bio industry's chicken farm's are very efficient regarding space and size however many more animals per human meal are required so more chicken suffer.
Another example I like is rennet and gelatine. Gelatine is by-product. Not one animal less is going to get killed because you eat a gelatine pudding because there's an excess of gelatine. So avoiding gelatine in order to increase animal welfare is inefficient. Rennet, per whole cheese, very little is required from the stomach of the (male) calve, and males are pretty much useless anyway since you need females for the milk. If you eat cheese with rennet regularly then a very low amount of calves die due to that. However technically, it isn't vegan nor is it vegetarian.
I'm sure we could produce more meat, raised in more sustainable ways, but that would involve people taking a major decrease in the amount of meat they eat. Modern levels of meat intake are at their highest levels in history. We can easily afford to lower are intake.
I'm also not a vegetarian, just know quite a few and have been around when the arguments come up.