jdw, horses in the wild with full coats don't do forced exercise that causes them to sweat through their coats.
So if you want your horse to keep his full winter coat all winter, that means you can't ride him past a walk under saddle, at least in my part of the world. Not an attractive option for many of us.
******ing the coat growth with blanketing is a partial solution, but I still had overheated horses after moderate work, so a blanket clip - clipping the hair from the head, neck, belly and flank area, but leaving what's normally covered by a blanket, is a better solution.
Clipping the horse completely, triple blanketing and stalling 12 - 16 hours a day is another solution, just not the one I would choose for my pleasure horses.