Based on this, I would bet that our current domestic meat animals would go extinct relatively quickly. There are some people who try to preserve various breeds in what are essentially zoos, but I can't imagine them operating in perpetuity.
1: https://www.livestockconservancy.org/index.php/heritage/inte...
But land has many other alternative uses. Until we value every one of those alternative uses less than we value preserving nature itself, we'll tend to use the land for something else.
Of course it depends on the land in question. Any land that is currently good for grazing and nothing should immediately be freed up once grazing stops being economical. Unfortunately, the land like that is not the most biodiverse -- it's rocky scrub, not forest -- but I'll be happy to see it added to the set of lands we don't want to mess with.
There’s also the whole infrastructure and supply chain dedicated to the breeding and raising of various meat sources. What will happen to them?
Our economy and workplaces are significantly different from what they were when horses were replaced by cars.
The 21st century isn't special and we weren't born a quarter century ago. The economy and our workplaces will continue to evolve, as they have since the beginning of time.
I think you're vastly underestimating how much work there is left to do.
The big issue is the slaughterhouses, which would obviously die out but which also may be the only big employeer around, kinda like the steel mills used to be. The farms raising feed are not likely to employ a lot of people.
Can you list them?