Upsetting as it is to some, there's probably no instance in which a cure to Kepral's syndrome would be attainable in Thane's lifetime. I'm no medical professional, but I imagine even having a medically advanced society doesn't necessarily mean finding cures for any and every alien illness is a walk in the park, given all the factors that might be involved.
Death is inevitable. Sometimes dying young is too. Still, with how his story was going, re/discovering reasons to live, stepping away from the fight, finding some peace, etc. I think it might have made sense, narratively, to allow Thane a death that didn't involve being run through. One that involved him living out what little time he had left with his son and whoever else he may be close to.