I got a great mane on my old horse, not by never touching it or brushing it, but by touching it and CAREFULLY brushing it every time I groomed. Lots of detangler, gently pick apart all knots with fingers, then CAREFULLY comb through until it didn't have a single snag. Same with the tail. If you don't allow knots and tangles to form in the first place, and keep everything slick with detangler, you don't get snags and breakage.
I also had her on biotin for her hooves, which may or may not have had any effect on her mane. This was back in the 90s, and I know there are more complete supplements out there now that have not only biotin but other important vitamins and minerals. Apparently it's pretty common for horses to also be Vitamin E deficient, which can potentially cause dry and brittle hair. One of the boarders at the barn gives her horse just normal human vitamin E capsules in his grain and he eats them fine. Whether or not they do anything for her horse, I really couldn't say, but he does have great hair.