Tons of vietnamese have horses in rural areas, I should ask them their secret to affording vet bills.

Well, I assume their horses will be less of a luxury pet. They probably pay less rent for the stables, treatment will be more DIY and the attachment to a working animal will be less emotional. I also don't know how expensive their vets will be relative to the general cost of living and the utility the horse provides. I imagine they'll have less overhead than our specialised clinics maintaining expensive equipment, dozens of specialised drugs and all the insurance and shit that goes with it in our system.

At the point we're at, a new horse with less health issues would be cheaper than all the money we've blown on ours, and she doesn't bring any utility. But we love her, and she's not suffering so badly that it would justify putting her down. As long as I can afford it, I will sooner invest in trying to heal what can be healed, manage what can't, provide the best life I can for her, own up to the responsibility I accepted when buying her, and enjoy our shared time.

But again: she's a luxury, maybe a step below actual sports horses with fancy lineages and tournament quality. I suppose if horses became more ubiquitous for transport, the affordability dynamic might shift, but for now, my remark should be taken as tongue-in-cheek and definitely won't hold up to comparison with working animals.

Who is going to stop the 1500 lb animal?

Ours is closer to 600, but voracious, headstrong and has shown a poor intuition for what's poisonous and what isn't. If she decides that a plant looks tasty, strength alone won't help you save her from herself. If you react in time, you can gently pull her head to the side, enough to turn her away but not so strongly as to hurt her.