What happened to the horse population once its marginal product became zero? They got turned into glue. Mostly, except for the lineages which could be retained

What happened to the horse population once its marginal product became zero? They got turned into glue. Mostly, except for the lineages which could be retained for entertainment value. I'm not at all saying that we'll see that happen: I'm speaking to the fact that people's marginal utility really can drop to zero. And although we might like to console ourselves with thoughts that everyone can end up a productive person with enough education and self effort, that's just a pleasant fairy tale. I've worked in service jobs before, and the people in the underclass there, though often genuinely nice people, aren't capable of being independently productive. Why can't they just form their own economy? Well, they will. In many places you can see what happens when the bottom rung run things: think the favelas of Brazil. Some level of cohesion with the broader economy, where it provides goods that for whatever reason are suppressed by the State in governed areas, coupled with rule by brute violence.