This was my first thought as well. The likelihood of those who inherit his copyrights to either squander them or being taken for a ride by the music industry is

This was my first thought as well. The likelihood of those who inherit his copyrights to either squander them or being taken for a ride by the music industry is very high. So him selling them himself allows him to negotiate better conditions and more money, which will do more to protect his legacy than just holding on to them until his death.