(1) If building human capacity has positive long-term ripple effects (e.g. on economic growth), these could be expected to swamp any temporary negative externalities.
(2) It’s also not clear that increasing population increases meat-eating in equilibrium. Presumably at some point in our technological development, the harms of factory-farming will be alleviated (e.g. by the development of affordable clean meat). Adding more people to the current generation moves forward both meat eating and economic & technological development. It doesn’t necessarily change the total number of meat-eaters who exist prior to our civ developing beyond factory farming.
But also: people (including those saved via GHD interventions) plausibly still ought to offset the harms caused by their diets. (Investing resources to speed up the development of clean meat, for example, seems very good.)
My thoughts: