Claiming Kissing Spine is “no big deal” in a symptomatic horse robs the horse of a chance to be treated for it.
If someone believes KS is not a big deal, they are unlikely to treat it. If they don’t treat it, the horse lives with chronic pain to the point of soft tissue failure like bilateral suspensories or even SI strains. Some develop behavioral issues and are passed from barn to barn marked as “pro rides”, “projects” or ‘quirky horses needing a soft hand/quiet ride’. Some develop behavioral issues mistaken for training issues and are cowboyed into programs to the point of learned helplessness. Others can’t reasonably tolerate the level of work asked of them and become dangerous to ride or handle.
Here is the thing about training and riding KS horses: you cannot train through pain. A horse in pain cannot reasonably yield its body to you in ways that hurt it. This includes under saddle, where KS symptoms are most evident.
Your scenario is the extreme end, the answer being somewhere in the middle. Many KS horses are servicably sound and can do just fine in LL riding programs with the right management and therapies.
But… If they are ‘servicably sound’, they are not actually asymptomatic, are they?