The fact that you don't understand what a self-contained movie is, even after having it explained to you, is telling. "Self-contained" doesn't mean, "nobody had any thought of perhaps a sequel coming some day." It doesn't mean there's no future or past tie-in media. It means you can understand the movie without consuming other media. You can watch The Godfather and never read the book or watch the sequels and understand the story.
The fucking movie ends with Vader flying away, mostly unharmed. His ship isn't even that heavily-damaged, which means that he's obviously coming back for a round 2. That isn't "self-contained" by any metric. Even if Lucas and his mates died off after Star Wars came out, you bet your ass someone would be making sequels right after the main villain walked away from the big battle alive and unscathed.
Star Wars is far from a self-contained adventure. The only people who think it is would be the ones who weren't listening to Obi-Wan as he was explaining things to young Luke Skywalker. It hints at a legendary past where the Jedi Knights used to keep things in order before the Empire. It hints at the story of Darth Vader betraying the Jedi and helping the Empire hunt them down. It hints at the future where Vader gets away from the Rebellion after the Death Star's destruction, obviously setting up a Round 2 for this fight between the Rebels and the Empire. If the original Star Wars was a self-contained movie, Vader would've died, or at least would've been disposed of in a way that looks like he died.
Even from the start, Star Wars was setting itself up for prequels and sequels, because people would obviously want to see how things were under the Jedi, how Vader betrayed them all, and what would happen with the good guys fighting Vader when he inevitably returns after the Death Star is destroyed, so even without Vader claiming that planet-busting is nothing compared to the Force, there's still tons of things to explain in future stories.
"Self-contained" means all the plot points are tied up; if the story ended there, most folks wouldn't have any problems. Having the main villain walk away unscathed is the exact opposite of that, since it implies that he'll be back and there will be future battles between him and the heroes on the road ahead.
The Force is central to the Star Wars movie, and it's not a giant question mark at the end. Star Wars establishes why Darth Vader said the Death Star is nothing compared to the power of the Force in the movie itself. It did this by showing an elderly man and his apprentice destroying the Death Star due to being in tune with the Force. The audience isn't left wondering, "Why did Darth Vader say the Force is powerful? Why did everyone make a big deal out of it? Doesn't seem like a big deal to me. I hope somebody writes a comic book or novel explaining it, because I am so confused." The giant space station explosion made it clear to everyone. There was no, "If this made no sense to you, just wait for the tie-in novel!"
This is how good movies work. They show you in the movie. There's no homework required.
Did you actually listen to his line? Note that Darth Vader didn't say the Death Star by itself is nothing compared to the Force. He said
"the ability to destroy a planet is insignificant compared to the power of the Force." That implies that the Force can do worse than just blowing up a planet, and that what we do see of the Force in the film is but a small taste of it.
So yes, if all you can do with the Force is hypnotize some morons or be a better spy and sharpshooter, people will start wondering why Vader said that blowing up a planet is nothing compared to the Force.
They might even think he's talking out of his ass, because the boys came up with something big that makes his outdated space magic religion look like shit. I'm pretty sure aiming better, hypnotizing weak-minded morons, and strangling someone whom you can see face-to-face is nothing compared to
disintegrating an entire planet with a push of a lever. The Rebels obviously saw the latter as a bigger threat than the former. Vader's been hunting them for years, but the Death Star is what forced them to put their big boy pants on and get all hands on deck to figure out how to crack this particular egg.
If Star Wars was just the first movie, and only the first movie, then the admiral who insulted Vader's religion would be telling the truth. Hence why the first major works after the OT centered around OP Force powers that yes, they used to not only explain what Vader was saying, but also plug up plot holes from Return of the Jedi. Because if you sincerely believe that a small fleet like that of the Rebels' would've beaten the large Imperial fleet above Endor on their own, then you need to read the Art of War again.
You've played way too many video games if you think there's an implied skill tree in the OT.
Not even. The way Luke progresses in the OT practically screams video game skill trees. To the point where again, he has a power boost, he has a rank level up (Chewie tells Han that Luke has become a full-fledged Jedi Knight, even though by that time, that rank is about as hollow as hot gas, since the Jedi have been gone for decades) and he even shows off his newfound power by choking people with the Force like daddy dearest and by mind-tricking Jabba's chamberlain the way Kenobi did.
You don't need to play video games to basically look at that and see that Luke's power progresses like a video game character. A power boost, a rank level up, and a skill tree-style upgrade where he's wielding the same powers that the previous two Force-wielding masters were using. They needed to portray Luke as a master Force-user on the same league as Vader or Kenobi, so they literally have him use Vader and Kenobi's powers.