Ok, let's get rid of the emotion of eating a pet (for those with hang ups about it).
We've been eating horses since before man painted on cave walls. If you spend enough time Western Europe you've likely eaten equine and not realized it (it had a tendency to turn up in sausages along with other meats) and it's not that uncommon in good restaurants in France and Italy (just look for "equine", but expect to pay more for it).
But all that aside, let's look at practicality.
1. Equine is one of the healthiest sources of domestic red meat. It's leaner than most and equal to the leanest cuts of any.
2. It's got amazingly high rates of iron and B-12 (and we must have B-12...sorry vegans, but's why you need supplements while we omnivores don't)
3. If you don't care for oily fish (salmon, etc...) the horse is the go to choice for omega 3. It's loaded with it (way over 300 mg/100 grams).
4. It's one of the few domestic animals who's meat is actually better when it's older. e.g. The meat from a 20 year old horse generally has a better flavor, etc... than that of a 5 year old.
With the exception of religious taboos (e.g. Judaism and Islam both have dietary restrictions against eating equine) it's only been in the last 100 years that the English speaking world went on a crusade against eating horse (it was still being eaten during WW I).
In general pigs are as smart as dogs (smarter than all the other domestic animals) and yet no one is bothered about eating an animal that is more intelligent than their cat or horse (and possibly their dog too, depending on the breed).
Now for the ultimate practicality. The TB racing industry alone (not counting any other source of "horse flesh") dumps about 5,000 horses a year on the market (OTTB's). That comes out to about 50,000 a decade. Add to that the number of horses that come from other sources and the number becomes staggering. The reality is that probably less than 20% (on a great year) of all these animals find a "home". Rescues can't handle the numbers and there will NEVER be enough people who want to AND can afford to take on the care of that many horses.
So, we kill off the rest (or the die from neglect or for what ever reason). Now we have to bury them to feed the worms, leave them out to feed the buzzards or expend an excessive amount of energy to cremate them.
Realistically it's a greater crime to let that much nutrition be deliberately wasted when it could go to much better use.
Personally it bothers me more to think that my horses we be recycled into a lower life form than it would to think that they'd provide good nutrition to another human (or even a dog). While I would be too keen on dressing mine out for meat like I would a dear or hog, a dead animal is no longer the "being" it was. At that point it's just a piece of meat and if you put a nice equine steak on my plate or cooked a nice equine roast I'd gladly eat it (and be healthier for it).
So rather than just have all this "excess" (and that's what they are....excess) horses, donkeys, mules, just be disposed of and go to waste I say put them to good use. Our ancestors certainly would have (and did).