But how were the idiotic Robinsons to know that?
They didn't. The writers of the show did. It's all made up.
A bit of HILL STREET BLUES realism here
Seriously? Category error. It was a 1960s adventure show
for children.
And if he did die in the process, wasn't that the original plan anyhow? Special guest stars aren't supposed to become de facto regulars, even if they're great at alliteration against uppity robots.
That's not what Jonathan Harris's credit meant. The characters of Smith and the Robot were not in the original pilot. It was decided that the series needed a regular antagonist added to the cast as a source of conflict and problems. Smith was always intended to be a permanent part of the show. However, the other actors' credit order was already determined by their contracts, which meant that Harris would have to be billed at the end. But it was considered inappropriate to credit an adult actor after several child actors, when he was getting paid more than they were (credit order and format is ultimately about who gets paid how much). So they invented the "Special Guest Star" credit to make Harris's last-place credit sound more prestigious, to compensate him for sticking it at the end.
After all, as I said above, '60s TV was more focused on individual episodes than on the larger whole. So episodes often centered more on guest stars, who could have complete story arcs in a single hour, than they did on the regulars, who had to stay unchanging from week to week. Thus, guest stars were often treated as more prestigious than the supporting cast, and presumably got paid more per episode to boot. If you look at the
season 1 end credits of Star Trek, for instance, you see that the main guest stars of an episode are generally billed above the semi-regulars like DeForest Kelley, James Doohan, etc. And in shows like
The Fugitive, there would often be an announcer introducing the featured guest stars at the start of an episode along with the main star of the show. So calling Harris a "Special Guest Star," even though he was a regular, was a way to make him sound important.
This was, I believe, the beginnng of what's now become a standard practice, giving actors a prestige credit at the end of the cast list preceded by "With" or "And" or some other special notation, which is considered second in importance only to the first-billed actor. A similar example happened with
Stargate SG-1. When Michael Shanks left the series for a year, he lost his second-place billing and Amanda Tapping and Christopher Judge were promoted to second and third, respectively. When he came back to the show a year later, they couldn't give him back his second billing, because it would demote the other two actors. So instead they gave him a prestige credit at the end of the main cast list, "And Michael Shanks as Daniel Jackson," which is tantamount to second billing.