I wasn't going to get into this, but for the sake of the OP's knowledge and any who want to know.
First. For those who want to dig up some "expert" who says the grains are fine (mostly oats is their go-to grain). There are plenty of them. I've met some. One, a VMD, who worked for a feed company and spouted off all these "experts" who lectured and presented evidence that he used and claimed he'd rather go with what they said. Having said that he could not refute the other medical evidence which refuted is love affair with oats and had nothing to say when I pointed out that he would fit real well with the tobacco industry's medical "experts" in the 60's and 70's who had their own studies which showed that tobacco products were not harmful to humans (where are those experts now?). Of course with people's health the gov't stepped in show the truth. Horses are unlikely to ever have that level of help.
Ok, this is going to be like a single frame out of a 2 hour movie. This is a large subject with loads of information. Just the information on the various kinds of starches in dealing with equine nutrition is much greater than what I'll give here which is tiny.
Grain. All grains are high in NSC (non structural carbohydrates). Horses only need small amounts of NSC and that's what they're designed to handle. Horses cannot digest long fiber (grass, hay, etc...) which is what their natural diet is made up of. In the equine hind gut (cecum and large intestine) they have an extremely large number of various microbes that consume the fiber. The byproduct of this is what the horse can absorb and use. These fiber eating microbes thrive in a PH7 environment which just happens to be what a healthy equine hind gut has. There is also preset a much smaller number of various that live on NSC starch. These microbes numbers are sufficient to deal with the low levels of NSC that the horse needs in it's diet. They thrive in a more acidic environment. Oats is the grain with the lowest NSC at about 40% (corn is the highest at 70%). Just the 40% that oats has exceeds what the equine digestive system is designed to handle (so think what happens if you add in the low levels from grasses and hays that also have some...or even worse...add in the NSC present in rich hays and lush grass...system overload). The microbes intended for dealing with the low levels of NSC have to go into a population boom. This will alter the hind gut PH and make it more acidic. That in turn will cause massive numbers of the microbes needed for fiber to die and be absorbed into the blood. This can have different negative effects on the horse's health such as laminitis. In short it can eventually have a fatal. result.
NSC's are needed for good equine health, but small amounts that nature has designed the horse to deal with. They are present in all grass and hay. The richer the hay, more lush the grass, the more NSC the horse will have to deal with. They certainly don't need more. Eating 5 lbs of bacon and 2 lbs of sugar a day is not going to kill you that week, but eventually it will adversely impact your health. Same with what we feed horses if it's not what they should be getting.
As for needing all this high quality rich feed, hay, etc.... in order for a horse to have a lovely, shiny coat and hair....
. That's a lot of hype. I'll post some photos of my mares who live on grass, low value "cow hay" and a mixture of beet pulp and copra. They have lovely coats and long hair (of course contrary to what I hear some "expert" claim, hair length, etc... is governed by genetics more than anything else, but their diet allows them to get the most out of what their genetics provides for). Note that Kit's tail is dragging the ground (due for a trim) in the photo of me on her and he coat is nice. Val's coat is also as nice as you could want. None of which was provided by rich diet. Just a proper diet that provides their nutritional needs, but doesn't disrupt the natural order of their digestive system.
Everyone is of course free to feed what they want. I just feel we should do it from a more educated and better informed perspective than the feed industry and traditions provide.