Everyone is asking "where's the market?"
If there is not market, horses won't be slaughtered: it's not like paperclips, where you can produce billions and let them sit in a warehouse until purchased - no contracts means no horses slaughtered in very short order. Good, right?
If the plant is built and there are no orders, it will sit empty, and likely bankrupt the evil company trying to make a living slaughtering horses....good, right?
And if they find a market other than Europe, well, there's the answer to that question.
But look at it from another angle; if slaughter and shipping to slaughter is made illegal, or if it is legalized and there is zero market, we still need to figure out what to do with the horses that may be left hanging ... homes, euthanasia? Who pays?
The rescues that can't keep their heads above water and horses fed while slaughter active? If people are so willing to absorb these extra horses, why is no one already doing so?
If it really is about saving the horses,
don't you think there should be clearer plans than "someone will give them a home, it's not my problem, I just stopped slaughter, so I'm going to wash my hands and go home feeling good and not think about those now in dire straights" in order to prevent issues when one or the other of your best case scenarios happens?
Otherwise, when it happens and there isn't a setup in place for horses that need a place to land, it will be the pro-slaughter side saying
"They did it themselves with their head in the sand, look the other way attitude."