How much of Murphy was left anyway? He has a rudimentary digestive system, which probably provides nutrients to keep his brain functioning, but how much of it is a brain/human tissue encased in that skull and how much of it is a processor. He has a human face, but that's probably stretched over a metallic framework, like a Terminator. I think, at some point, what's left of Murphy's brain is going to age and go through cognitive decline, resulting in it being completely replaced by a computer. The same goes for his digestive system. There will probably be nothing left of Murphy, but a program that thinks it's Murphy.
What's left is Murphy is basically his badly damaged brain and the minimum organs necessary to keep it alive. The second movie alleged that even his face was artificial ("They made this to honor him"), but that seems unlikely given that it still has a bullet hole in the forehead. He may have just said that to his wife to help convince her Murphy was dead. (Conversely, the 1988 Marvel Productions animated series depicted Robo as much more human, capable of eating, drinking, catching a cold, etc., and even able to remove his headgear like a motorcycle helmet rather than having it bolted to his skull. It foreshadows the 2014 reboot where Murphy wasn't actually killed before his Robo-conversion.)
In principle, RoboCop was governed by a computer program and Alex Murphy's brain was merely a supplement to facilitate the control of the body, since they had trouble cracking that problem cybernetically (e.g. ED-209 going out of control). The builders never anticipated that any of Murphy's memory or personalities would survive in his badly damaged cerebrum; after all, he was brain-dead when they got hold of his body. Most
RoboCop productions assume that Robo is just Murphy in a mechanical body, and even the original film seemed to treat him that way by the final act; but I'm partial to the interpretation of
RoboCop: The Series that RoboCop's personality is an amalgam of the RoboCop programming and the fragments of the deceased Alex Murphy's memories and personality that managed to survive. He's both Murphy and not-Murphy, which is why (in the series) he hides his identity from Murphy's family, feeling it woud hurt them too much to know.
So yes, Robo's functioning is dependent on the brain, and that brain is in pretty bad shape to start with. So it's questionable whether Robo even has the kind of life expectancy that Murphy would've had. On the other hand, since his life support systems don't have to take care of an entire body, maybe that would reduce the strain on the brain, heart, and whatever else survives and allow them to function longer.
Also, if Robo's personality is an amalgam of his programming and Murphy's memories, then it could be that the memories and their personality influence would live on in the cybernetic systems if a way were devised to replace the organic brain with a technological substitute. Though that seems iffy, since it's the interaction between the two that makes Robo who he is. It would be like replacing a hemisphere of a living person's brain and expecting them to be the same. Maybe it could be done if the replacement were gradual enough, a Ship of Theseus thing.
