I cannot speak for Elon, but I have my reasons why I am not focused on what you suggest.
First of all, cancer is not a single disease; it is a suitcase descriptor for many types of disorders, each with their unique cause. So, solving cancer in one fell swoop is just not feasible, and it is induced and aggravated by ourselves. Despite that, many scientists are working to find cures, and significant progress is being made in certain areas of cancer analysis and treatment.
Which brings me to my second point. The evolution of humanity is not preserved by increasing our individual life-span (in qualitative ignorance), but by the strengthening of our renewability. Indeed, our timely death is vital (no pun intended) to the next version of us to live stronger. Our timely renewal prolongs the sustainability of humanity.
Which brings me to my third point. The systems we build to manage human performance are fundamentally flawed and the source of our anthropogenic demise, no matter where we end up living (extraplanetary or not) or what diseases we will face.
Hence, we must race to strengthen the operating system of humanity. A system that must begin to embellish and propel our meaningful differences over our stifling commonalities. So we keep expanding the ingenuity and capacity at the fringe of our equilibrium with nature, and in accordance with its evolutionary rule.
So, for us to live as healthy as possible and avoid cancer we best care to comprehend and subjugate to nature’s rule to strengthen ourselves. Life is indeed that simple.