"The Motion Picture" sounds stilted, but I can see why they'd want ST in the title and something else to distinguish from the show. But it was aiming more for cinema than "the movies," hence the semi-stilted choice.
I think it was still in more common use back then, since there were more people around who'd grown up in the early days of film when "motion picture" was used more often than the shorthand "movie." It was considered a classier term, while "movie" was seen more as slang. At least, that was my impression.
I think Superman would be a stronger titled just "Superman." There was no recent tv show to distinguish it from and "the Movie" seems anti-climactic, both in meaning and rhythmically. "Superman": wham! a great word, sound-wise. Supermanthemovie kinda peters out. And with all the promotional work, everybody knew it was a movie.
Actually there was a current TV show featuring Superman, namely
Superfriends. Plus the George Reeves
The Adventures of Superman was in pretty constant syndication back then. And of course more people read comic books back then because the distribution monopoly hadn't happened yet and thus comics were still widely sold in drugstores and department stores rather than just in specialty shops where only committed comics fans would ever know they existed. So Superman was a regular media presence in several forms; it simply isn't true that the movie was filling some long-standing void.
And really, I don't understand the "everybody knew it was a movie" argument. I mean, people know
Batman: The Animated Series was an animated series, but I've never heard anyone object to the title. And there are plenty of other TV shows based on movies called
The Series. Such descriptors are useful, because it's not just about "informing" people of what something is, it's about distinguishing different branches of a franchise for general and long-term reference. Certainly it's useful in retrospect to talk about
Superman: The Movie or
RoboCop: The Series or the like so that it's instantly clear what subset of the franchise I'm referring to. It's certainly handier than talking about "
Star Trek" and having to add words to specify whether I'm referring to the original series or the 2009 movie. This kind of subtitle is useful and practical and I don't understand the objections.
Anyway, I grew up hearing it called
Superman: The Movie, so the modern tendency to refer to it as just
Superman sounds weird to me. Different people perceive things differently.