I've just made the decision to back off riding with my Haflinger gelding. I'm not totally giving up riding, we'll still do off-property trail rides if we get invited along, and if the barn has a play day or something. There's nothing wrong with him physically, he's a gentleman under saddle, but as a rides and drives, I have a feeling that he'd much rather be pulling his cart in harness than trying to put up with me mucking around with multiple saddles in multiple disciplines. Which was frustrating me too, I felt I lacked motivation and direction that I think driving could bring back because it's super fun with a well trained driving horse.
I have a mare who has been "intermittently gimpy" the last few years and we've been unable to pinpoint the exact problems short of a very extensive (and expensive) full body exam. It's possibly something in her back, as she pins her ears and tried to buck at a gait faster than a walk, and only does it with a rider on her back. So she doesn't get ridden, I take her out for walks, and as I have a spare harness, ground drive her to give her a "job" to do (she'd be perfectly happy as a pasture puff, but I don't really have a pasture -- big corral, sure -- and I know she likes going out and moving around and at least she's got something to keep her from getting bored). Maybe someday, we'll take the next step and she might end up driving too.
I know other people at my barn who doesn't ride their horses. One is the woman who is my "mentor" ( she almost feels like an older sister, or an aunt). One of her Arabians is older and was diagnosed with Cushings a few years ago. Her other Arabian is still rideable, but she feels he's not quite up to her weight so she has a lovely gal come out and ride him so he gets his stuff done. Another person is a woman I think got a little spooked by her horse's high jinx a few years ago and stopped riding. She spends her time grooming him.
People don't have to ride their horses. Although I think when it comes to boarding, the costs are often high enough that someone should at least be doing something with their horse, be it riding, driving or in hand trail walks.