There are three potential responses to my argumentation which I would like to tackle. One response is that the conditions animals in factory farms suffer are indeed worse than death. I do not believe this is true but even if it is, it is entirely possible for meat eaters to restrict themselves to free range meat which gives the animals it is taken from a reasonable quality of life. If this means eating meat less often, so be it.
The second, better response is that morality if absolute, not relative. Even if eating meat is better for animals than not eating meat, that does not make it moral, just comparative less immoral. For example, torturing and raping someone is worse than just torturing them. That does not mean torture is moral, only that it is comparative less immoral. Hence, both eating meat and killing animals as well as not eating meat and causing the non-existence of potential animals are both immoral. The moral course of action would be to ensure animals exist and to not kill them, for example by donating money to animal sanctuaries
The issue with this line of argumentation is that it presumes acceptance of not only a negative duty to not harm animals but a far stronger positive duty to ensure they live and live well. If you accept we have such a duty, than vegetarianism can indeed stand up to my objection. But, the consequences of accepting we have such a positive duty are that we need to intervene wherever animals are suffering, even in nature at the hands of other animals, in order to prevent that suffering. Given that most people would not accept that we have a duty to intervene in nature in such a way, I doubt most people do believe that we have a positive duty of care towards animals and hence I believe that this specific defense of vegetarianism is not sufficient to persuade the average informed voter that they should be a vegetarian.
The final response is that talking of the preferences of non-existing beings is ludicrous. For example, arguing that women should always be pregnant else they deny non-existent children their right to life is crazy. So is arguing that not eating meat denies non-existent animals the right to life. Our aim should be to avoid killing and if that means less animals so be it. The problem with this response is that even looking only at the preferences of currently living animals, insofar as we can assume animals have preferences, it is hard to imagine that any species of animals would prefer a world where their species did not exist, or existed in far reduced numbers, to one where they did exist but some of their kind who would otherwise not have been born are killed by humans. Hence, even ignoring the preferences of non-existent animals my argument still holds.