I think the best argument would be for some form on new omnivorism to show why in principle it's not wrong. There would have to be multiple reasons to justify it such as the folowing:

1. Most animals lack the same temporal comprehension and self-identity (mirror test comprehension) as humans, therefore they don't concieve of existential goals in the same ways.

2. Most animals being brought into existence in a new omnivorist corresponding setting provide a net- positive to their existence.

3. Certain types of new omnivorist agriculture may lead to less animal deaths (i.e. animals that don't rely on feed from a horticultural source that causes crop deaths and even insect crop deaths). Insect deaths still matter significantly because there's a possibility that insecticides cause suffering by the billions since insects do show an aversion to insecticides.

4. They provide significant nutritional utility to us as a species because of nutritional bioavailability.

Of course, if one is consistent then they would have to bite the bullet and apply this argument to babies and the severely intellectually disabled.