You're going to harm insects anyway. Do you know how many small mammals and insects get killed every time a field is harvested?
Presumably someone who is vegan for ecological (rather than 'animal rights') reasons will accept a few accidental deaths in exchange for a significant reduction in CO
2 production, water usage, runoff and so on. In other words, some incidental insect and small mammal deaths in exchange for a much healthier global ecosystem overall, including oceans and waterways. For the animal rights crowd, the fact that the animal deaths are an unfortunate accident of critical food production, as opposed to animal deaths being the immediate goal, probably matters as well.
I'd also question that logic, because industrial (not grazing) livestock eat industrially farmed grain and grain by-products as well. So one one hand you get the number of animals accidentally killed when harvesting grain (which is incidental rather than inherent, and something you
could try to reduce), and on the other hand you get animals accidentally killed when harvesting grain
plus the animals intentionally killed for meat. So the vegan guy is, on average, still involved in fewer animal deaths.
Regardless, if China starts eating beef at the same rate that Americans do the planet is just screwed from the CH
4 and CO
2, and freshwater reserves are in trouble too. I'm not saying you should stop eating beef altogether, just that current consumption levels are already unsustainable and it's going to get even worse.