You are right. It is actually more likely the murderer is dead than not. This is a simple observation because lots of people associated with the investigation are dead too. Simply beacause time has passed. Christer Pettersson is dead. Hans Holmér is dead. Lisbeth Palme is dead. 33-åringen is dead. Claes Zeime is dead. Axel Morath is dead. Lars Tingström is dead. Skandiamannen is dead. Ebbe Carlsson is dead. And so on, and so on..
Oh, I had already completely forgotten about Ebbe Carlsson. The Swedish Wikipedia article was I nice refresh of my memory and now I understand again why I had read every article back in the days. Better than any fiction... (The English Wikipedia does not mention anythig of the dozens of scandals and weird affairs he was involved in.)
I'm not a Swede, but I happened to be in Sweden as a trainee in Sweden just a year after it happened. For a couple of years I read every article I saw. But now that so much time has gone, I don't read any article any more. Not even this one, I was just curious what people comment. It is not unlikely that the murderer has died in the meantime. The same holds for anybody who might have (had) knowledge what exactly happened. The credibility of any new witnesses after so many years cannot be very high. If someone confesses and can offer unique proof/insider knowledge I will hear about whithout reading such articles. Or if credible documents are found in some secret service's archives that they were involved. But it's not anything likely enough to happen that I could still be interested.