The Osaka Tower is what is known in fiction as a Red Herring ; in other words, a seemingly important element that serves only to distract the viewer from the truth. A traitorous Chekhov's gun if you will. In Glass, the Osaka Tower is the mother of all Red Herrings at it dupes both the viewer and the characters trapped in the narrative.
Every character in Glass' setting is aware of the Osaka Tower and its relevant characteristics : it's massive, it's nearing completion, and heavily mediatized. Once Mr. Glass puts his plan in motion, he puppeteers everyone around to make them believe he's staging a fight between The Overseer and The Beast at Osaka Tower, leaving a note stapled to a newspaper showcasing the building. He even gives a perfectly good reason for this strategy : the media attention around Osaka Tower would out superhumans as a component of reality. That is why The Overseer and the psychologist are so adamant about foiling Elijah's plan and keeping The Beast from leaving the hospital's grounds.
However, Mr. Glass never once seriously thought about holding a final showdown at Osaka Tower. As he explains to his mother, he was staging an 'origin story', not a 'limited series' in which a final battle would be necessary to fit into narrative tropes. The tower is but a smokescreen to goad every character into playing their cards at the hospital and outing themselves to the world. It could be said the battle at the asylum was in itself a final showdown, but as it resolves inconclusively, I wouldn't say it fits Glass' universe's definition of a 'final showdown'.
Now, from the viewer's perspective, it seems only logical to hold a final battle at Osaka Tower. First, a newscaster can be heard at the beginning of the movie highlighting the massiveness and celebrity of the building. A savvy film-goer may then be tricked into believing there will be an obvious showdown at Osaka Tower, because it fits within a typical 3-acts story structure, both within Glass and within the Unbreakable trilogy. This guess is reinforced by Elijah's note on the newspaper. Notice that the journal describes the Osaka Tower as a 'true marvel', hearkening back to stereotypical Marvel movies where a gigantic brawlfest is expected to take place, preferably in an epic setting.
Of course, this illusion of understanding is shattered when The Beast and The Overseer are both killed by the Clover Society. The Osaka Tower was never meant to be the final showdown's location, because Shyamalan had already decided it would not be this kind of movie. It was just a shiny beacon to lure away the viewer from the final twists. As the Huffington Post puts it :
“Glass” teases the big Osaka Tower calamity so hard you might believe it’s actually going to happen. Without a third-act battle, how can this saga hew to the comic-book blueprint that Elijah routinely references? Even Elijah’s mom (Charlayne Woodard) knows that a culminating “showdown” where “all the skills are revealed” is pure formula ― something we’ve seen over and over and over in Marvel and DC leviathans. (A magazine headline touting Osaka as a “TRUE MARVEL” is no accident.)
