So what? We don't need as many. It doesn't make life any worse (in fact, probably better) for the ones we have now. Those pre-industrial-revolution horses would have been dead by now of old age if not turned into dog food.

That's the whole point of the parent argument. What do we do with people who's work output we don't need anymore?

We don't do anything. We'll have to take care of them, which is where things like a basic income help. It won't happen all at once instantly, and it's not like our only option is to grind them into soylent green. Horses may get turned into food, but that's because they're horses and not people. We eat animals, so it's not very compelling to say "oh no, we ate all the horses or turned them into glue". Who cares. The point is there are horses now and there will probably continue to be horses. Many of them with a life of mostly leisure and good care instead of being worked to death. Sounds good to me.

As long as we're all clear that you're arguing for a significantly reduced human population, I guess I don't have anything to add. I'm pessimistic that the transition between our current populous life of toil and this idyllic future will be peaceful or pleasant.