As a first time viewer of a now-50+-year-old movie, I find it necessary to offer a number of concessions: 1) I cannot expect the visuals to meet contemporary standards of "realism" (limited budgets + limited technology); 2) I cannot expect characters to develop in ways that are anything but consistent with a 1950s cultural milieu; and 3) I must accept pseudoscience. That's more or less how I came into the film. After all, if you watch Forbidden Planet, you'll become aware of the limitations of the cinematic medium during the 50s, the rampant, almost "rapey" sexism that was all too common during the "glory days," and the laughable nonsense that passed for "science" then (and still passes for "science" today).
And yet, for such a campy film, Forbidden Planet does something that only the best SF films do: eloquently visualizes and explores the science of a future world, even if, upon further inspection, much of that science is impossible, unexplained, or downright false (it was the 50s, after all). The
opening scenes, for example, imagine a future in which FTL is possible, but not in the fanciful and convenient way of Star Wars, which wouldn't appear for another 20 years. Rather, the crew reminds us of two important things: traveling to other stars takes an extraordinary amount of time and deceleration is not a "cakewalk." Navigators must set the deceleration process on a "timer" and climb into the anti-gravity pods to wait the process out. Nothing is every quite explained. How do the pods work? Why do they turn strange colors and "disappear"? We just don't know.
2001: A Space Odyssey (1968) provided us with two extraordinary sequences inside massive, moving sets, the object of which was to mimic for the audience how artificial gravity (and stasis pods, for that matter) might work on a visual level. The scenes are beautiful, if not a tad dated, and perform exactly the same function as the opening minutes of Forbidden Planet (we're meant to accept the unexplained monoliths and the Starchild). Alien (1979) and parts of Aliens (1986) are similarly focused on the technological mechanics of the future. The former contains no dialogue until nearly 6 minutes into the film, instead focusing on a) the computational abilities of the Nostromo (which haven't aged well), and b) the long process of waking from stasis. Aliens reverses this imagery by showing the decrepit condition of the Nostromo's escape shuttle, which salvage crews must cut into before they can extricate the sole survivor of the previous film -- a person nobody was expecting to find anyway. Rather than focusing on how technology has "advanced," the sequence focuses on how the very technology that made the previous film possible has remained static in time, providing the necessary jolt of reality that Ripley will need to reach the next stage as a hero. The result? We're drawn into the world so we can more easily take that leap of faith when the seemingly impossible alien(s) show up.
I'm not sure why this trend apparently died off. Budgetary reasons? Were audiences disinterested in the extraordinary details many SF writers/directors put into their work because of pacing concerns? Your guess is probably as good as mine (unless you're an SF film scholar and have answers). One thing is for certain: it's a fascinating and illuminating SF trend. Perhaps we'll get something like it again one day...