Not sure what the solution would be that is proposed by the legal experts but it seems to me that we already have a system for dealing with that for the office of the president in the form of vice presidents etc taking over if they die. Not that you should have to have an entire chain of people ready to take over for every SC justice but rather, if one dies or retires or whatever before their 18 years is up then a replacement can be appointed to finish the remainder of their term.