Is it high ringbone or low ringbone? We had an older upper level dressage schoolie that was diagnosed with high ring bone, she did really well on a combination of Aspireaze (horsey aspirin) and monthly or bi-monthly shockwave treatment. She still had off days, especially in bitter cold or wet weather, but was perfectly happy to do her job. It became more of a mechanical lameness than a painful one after the bone remodeled quite a bit (the pastern area just didn't have the range of motion it used to, but it no longer caused her pain). And she no longer had to be on NSAIDS. She was able to continue being a third level schoolie for over five years after diagnosis, then she got to be a pasture ornament ;). Because of the ringbone she wasn't competition sound, but she was servicably sound and loved her job. She taught a many a rider to be a better rider.
All hope is not lost!