For a lot of them, the answer is most certainly yes, and if humans were in their positions, they'd say so too.

Then I think the natural follow-up experiment would be to imagine if a dog you know was born for dogfighting, and that you can be sure it will never escape. Would we rather it just not be born? If not, we just need to argue that being bred to dogfight is preferable to, or on-par with, being bred to be trapped in a cage and eaten.

I never understood that logic. I don't see why more animals (or humans) existing ought to be considered a good thing?

That isn't the core of the argument; the point is that there is no good outcome for most of those animals. The choice is exist, live and be slaughtered for food vs. never being born.

Not an enviable choice, but it confounds some of the simple ethical arguments for, eg, vegetarianism.

But I don't understand how it is relevant to what we should do?

Our goal is to minimize the amount of suffering we are responsible of. In that respect it is obviously better to not breed animals than to breed them and make them suffer.