whether or not the astronaut is willing to take the risk, kidney stones can be so painful that someone suffering from them could potentially jeopardize the miss

whether or not the astronaut is willing to take the risk, kidney stones can be so painful that someone suffering from them could potentially jeopardize the mission from incapacity or increased risk of making a mistake. Even if the astronaut were personally signed up for a suicide mission, there's a lot of incredibly expensive and difficult science and exploration to be done.