I owned a show horse with only one eye for many years. We retired her and only rode her on trails, but she did very well. The only thing I ever had to really watch was the kids being on the ground on that side. She might not see them and hit them with her head. Otherwise, she adjusted very well. We even drove her with a cart. She lived to be 32 years old and then lost sight in her other eye. We had her for another year, completely blind. I don't recommend this for very long. It is just not fair when they are this old. We loved her and she was a great horse despite her disability. Don't underestimate the horse's ability to adapt and thrive.
Me and another girl here are sharing a horse right now. If she sems this thread she will probably reply as well. But the Appaloosa mare is pregnant as of right now and she blind in one eye. And when we first bought her we rode her by a BIG fan on her blind side and it didn't bother her a bit. We also rode her between to closely parked cars and she didn't mind. But she has been blind in that eye for over 9 years easily. I also think she may be going deaf or something. I was on her blind side and was talkign to her and when I touched her face it scared her to death. So make sure you always approach from the non blind side and work your way around unless you make sure they hear you loud and clear. I took one of the pictures standing directly behind her rear. And the one with me riding. I was riding in the dark and in the snow. And it was only the 2nd time I had ever ridden her. And she did fantastic even bareback.
There is also to be known completely blind horses to compete and horses that are blind in one eye to still compete in shows and such.
No, I don't think it's fair to them either, but I can understand how attached you can get to them.
I've seen a few people try to find homes for totally blind horses. I don't think that's fair either, to take them from familiar surroundings and put them in a strange place when they are totally blind. Unless of course it's an emergency or a rescue.
you can do anything with a blind horse. With enough training and trust.
And here are some rescues for blind horses. Flurry's Hope HorseNet Horse Rescue
I know there is more and there is also an awesome video of a family who started a rescue specifically for blind horses but I can't seem to find it.
I owned an appy that went blind in her right eye - she was about 20. The eye had that cloudy look, but otherwise was healthy. She was totally fine and you could ride her and you'd never know she was blind.