Higgins, T.C., Rick, and Magnum are an odd bunch of fellows, and here's a fan theory to explain why Robin Masters is a character on the show until he isn't. First off, we are told on the show that T.C., Rick, and Magnum are friends from the Vietnam War and that Higgins is the estate manager for Robin Masters Hawaiian property. In the earlier seasons of the show, Robin is a real person never fully shown on screen and by the end of the series, he is less and less discussed as a tangible human being. Eventually, Higgins in the series finale confirms to Magnum that there is no Robin Masters because he is Robin Masters. This reveal is mostly a neatly sewn together plot device if only there wasn't so much room for theorizing.
My theory for the show is this; Robin Masters was a real person who was killed off by Higgins and Magnum a short while before the storyline of the series began. Magnum was the compound's security expert and Higgins was the property manager, together they conspired to kill Robin Masters and allow Higgins to slowly assume his identity. T.C. is well aware of this happening and probably had some part in disposing of Robin Masters's body. This would explain why T.C., a former soldier (a career not known to rake in the riches) could afford to buy his own helicopter company at such a young age. As for Rick, Higgins may have allowed him to run the clubhouse in exchange for silence on the Robin Masters situation and to use Rick's mob connections to keep prying interests away.
As for Magnum he often had visitors on the estate, but no one who knew Robin Masters personally. He and Higgins also might have had a constant flow of house guests as a means of being able to keep up the appearance of normality on the estate as well as being able to have fresh alibi's/ scapegoats. This might be why neither Magnum or Higgins ever left the estate for too long of a time and why neither of them had long term love interests that lived on the property with them, despite both being in their prime.
A curious thought to further explain matters is that it is not out of the realm of reason for Magnum and Higgins to have been romantically involved. This could have been the catalyst for their removal of Robin Masters. A lonely former Navy officer and a far from home steward spending large amounts of time together, enjoying the finer things in life. This could have been the catalyst for Higgins and Magnum to develop some nefarious plans to maintain the lifestyle they became accustomed to. The two could have planned the whole thing out while intertwined in each other's arms.
The series ended when Robin Masters was no longer sought after and Higgins could safely assume his identity without suspicion. On the day of Rick's wedding, when the house was full of visitors, Magnum and Higgins had a conversation regarding Higgins as Robin Masters seeming to cement the rumor themselves semi-publicly. This would also explain why no matter how heated the argument between Magnum and Higgins, Magnum was never fired and there was never any other full-time staff at the compound either. Oh, those love birds!
This is a theory that seems to hold up for the most part, if you completely discount Higgins using the alias of Robin Masters as his pen name. The support for the separation of the two as a single person comes from Masters having a well known public persona, something Higgins totally lacks as the frugal homebody he portrays. With Higgins keeping out of the limelight, he could have emerged as an aged Robin Masters by the time the series ended.
Another far fetched thought about the series is that it was a well-integrated form of communist propaganda into Western society, with Tom Selleck as a solider turned actor to throw off suspicion to the troublesome truth. IDK, I'll leave it up to the reader to make up their own mind on the subject.