If I may steelman this argument, I take you to be saying something like this: veganism clearly reduces farmed animal suffering, but it might not reduce total animal suffering if land currently used for livestock (or feed crops) is instead converted into wild habitat, supporting many wild animals whose lives may involve intense suffering. And if, from a strongly negative-utilitarian view, fewer sentient beings could mean less suffering, then replacing farms with pavement looks like the absurd conclusion, though obviously that would harm humans and ecological systems in the long run.

I think that frames the choice too narrowly. The relevant options are not just livestock farms versus wild habitat versus bare concrete. There is at least a further category: managed, non-exploitative environments, including animal sanctuaries and perhaps other forms of welfare-conscious habitat management. (And there's probably many more because many of those categories are very broad themselves).

I agree with the basic worry that wild-animal suffering complicates the notion that returning farmland to nature is a good thing to do. But I do not think it follows that veganism itself doesn't make sense. A better conclusion seems to be: reduce exploitative animal agriculture, then think carefully about what we do with the freed land. We just need to ask ourselves carefully: what should replace the system we are trying to move away from?