(or "Death Is But A Door")
So what, exactly, is it about the Metal Men that makes them such an enduring property? In a lot of ways, they seem very similar to many of the other quirky, off-beat characters that DC created in the Sixties as they tried to capture the youth culture, and their storytelling engine reflects that. Unusual villains (like Chemo, the Gas Gang, and a giant radioactive version of their creator, Doc Magnus)? Sure. Quirky characters that don't always get along, led by an irascible mentor? Yep.
A slightly different take on the "public hero" motif, with the Metal Men sometimes loved and sometimes feared by a fickle public? Check. But all of these are shared by the Doom Patrol, the Fantastic Four, or even the X-Men. There's got to be something unique about their storytelling engine, something that keeps it going through all these years.
They're all robots, and when they get killed, Doc Magnus just rebuilds them? Yep, there it is.
And it doesn't just keep them going in the literal sense, either. The Metal Men, as characters, endure because Doc Magnus keeps finding a way to put them back together after they sacrifice themselves fighting the enemies of humanity, but the concept of the Metal Men stays strong because removing death from the equation of their storytelling engine opens up the potential for some truly strange, genuinely memorable stories. Nothing really stops the Metal Men, allowing the writers to get them into situations that no other hero could get out of and exploring the nature of heroism in some decidedly odd ways. Metal Men get trapped in space, become radioactive, turn evil, blow up, blow themselves up, and through it all, writer Bob Kanigher knows that he has a "backdoor" out of these plots in the form of Doc Magnus and his robot-repair skills.
Of course, removing death from the equation has some downsides as well as upsides. It's hard to really feel concerned for one of the Metal Men when they wind up in a life-or-death situation, because previous stories have demonstrated that there's really no such thing for them. No matter how devastating the injury visited on Tin, Lead, Gold, Iron, Mercury, or Platinum, they'll be back again by the next issue right as rain. The only way to induce real tension is to go after Doc Magnus, something the series does regularly. You can almost think of it as a logic gate; the potential for death opens up tension, but at the cost of closing off storytelling possibilities.
Readers tend to thrive on tension, though, so the "unkillable protagonist" remains confined to a relative handful of storytelling engines (Resurrection Man, arguably Metamorpho, and Robotman of the Doom Patrol tends to be pretty tough to get rid of.) But those few series provide something genuinely different for the readers, a glimpse into a world where all the rules are changed...even the most fundamental rule of life.