I think a lot of people who are tied up in this grain free stuff for their pets (or even themselves, looking at you keto diet) are just trying to do the best they can. When we look at the discussion from that POV it reminds me of an economics class I had a while back. I’ll try not to butcher it.
The prof demonstrated an example regarding volunteer service. Let’s say you want to volunteer your time to build birdhouses for an endangered species of bird. You can:
A.) Volunteer your time to build the birdhouses yourself.
B.) Donate money that goes towards hiring a carpenter to build those birdhouses.
Most people prefer option A because of the altruistic feeling it gives them. We get much more satisfaction volunteering our time and performing the task ourselves than donating money.
However, option B is likely to provide a higher quantity of better built birdhouses.
I apply that line of thinking towards commercial feeds for my animals. Yes, I can likely spend the time and energy building a diet for my horse out of “rough” materials like beet pulp, vit/min supplements, etc. I will probably gain more satisfaction knowing I created that diet! Look how good I am to my animals! But I only have a passing understanding of equine and feline dietary needs and think it is probably safer to leave these formulations in the hands of scientists and professionals who have the proper expertise. So I choose commercial feeds that meet my horses needs.
This should all be taken with a grain of salt. If you really have the knowledge that it takes to DIY equine feed (as @JB does) go for it!
If your horse has a specific allergy/need/whatever and your vet/a qualified nutritionist think it needs a certain diet, by all means follow that diet.
But as an average equine owner, most of us are probably best off leaving it to the professionals and by professionals I don’t mean your riding instructor or barn owner. I mean the scientists formulating feeds backed by science.
As owners, we are advocates for our animals. I try my best to arm myself with a wide range of somewhat shallow knowledge. Then when I am out of my depths, I bring in the professional. I would love to have a deep and wide range of knowledge but there are only so many hours in the day.