You can feed your horse nothing but sweet feed for all I care personally
You can make your own mix of 40% whole oats, 30% barley flakes, 20% cracked corn, 10% wheat brand (horses LOVE wheat brand like a child loves candy) and bind it all together with molasses. Will they die from it? Not for some time, but eventually there will be an impact.
You can argue the point of the amount makes a difference and of course it does, but the amount of oats small enough to have no impact on the hindgut would be too small to provide what little nutritional value you hoped to get from them in the first place.
What you can't change is the fact that you have to feed enough of anything to get a nutritional benefit from it (one blade of grass doesn't help much). With any grain, the amount needed is going to put more starch in the cecum than a horse should technically have (and more than it needs to have). This will always have the effect of changing the ph in favor of the microbes needed for dealing with this extra starch. That ph change will always be to the detriment of the microbes needed for digesting fiber (hay, grass, beet pulp, etc...). Those are the facts. Talking around it and saying "I don't feed more than what's needed" isn't going to suddenly change what happens in the cecum
. If it did you'd need to try it with humans too. Most of the world's health problems will be solved.
Humans do not process starches like horses do, so what oats do for us has no comparison for equines. We need oats primarily as a water soluble fiber (probably it's greatest human health benefit). That's not what they are for a horse.
People often have underlying health problems that go unnoticed or undetected for years, even decades (e.g. you liver can slowly deteriorate for many years before you notice any signs or before a Dr runs a blood test that tells you something is wrong....the damage was being done earlier, but no one knew it). It's a bit like horses eating acorns in season. It doesn't kill them so it must not be too bad (like grain, some of us know it poses a health risk...just happens to be a greater one), but over time the damage to their kidneys (if memory from class serves me right it was the kidneys) does build up and it can eventually result in death.
How often does anyone (or their vet) check the ph level of the horse's cecum? If you're feeding grain you might want to seriously take a course covering the equine digestive system. Find out what the ph is suppose to be with nature's diet (no grains, low NSC, etc...) and then have it measured with your preferred diet.
Of course I'm picking on grains, but that is what this thread is ultimately about. There are other dietary issues with how we keep horses. Grains just happen to be one of the larger ones. (if it was about extruded foods used by the feed industry I'd be picking on them
Everyone is free to feed what they want. No one has passed a law forbidding anyone from feeding anything that has traditionally been fed before (including all the cracked, ground or whole maize a horse can eat). No matter how unhealthy, it's still legal. Killing a horse with kindness has yet to become a crime
so don't worry about it.