The reason senkus cracks are gone now of all times, even though senkuu has been petrified many times, may be because he's finally achieved the dream he and his father held for so long. The cracks represent senkuu's perseverance, his dreams, and the memory of Byakuya. So when Xeno does give him this time alone in space to mourn, not only is he achieving his dream of being in space, but senku is also allowing his father to rest in his mind. He has a moment alone to process, and shed these scars, physically and mentally. But that's just me I could just be reading into it too much ¯\_(•‿•)_/¯