Can you prevent break down in a performance horse?

it depends what you call a "breakdown." an older horse who is just getting slower and slower, isn't really considered a breakdown to me. but a 12 year old TB who is prestigious in hunter/jumper, and suddenly just refuses jumps, or in other terms "stops being good", i dont really think that can happen. i just think that would be behaviour issues that can be worked through, or a mentality that can be broken. i am a strong follower of parelli (however you spell it) and it might back up my answer...if my answer makes any sense at all