Elon Musk has done what he has done for decades. For some people working very hard is sustainable and for others (probably most) it is not. I much depends on constitution and also what one values and enjoys in life. I have personally also "worked" very hard since the 1970s, I enjoy doing so, it makes me feel good, and I plan to continue working hard into the future. One of my mathematician colleagues (Barry Mazur) is 80, and is one of the most mathematically productive people I know. For some people there is a cumulative benefit to long hours of hard work... for others it is surely a total disaster. Life is so complicated.