If you think about it, you got Kakashi right who is like nigh Bijuu to Bijuu level at his peak (in Naruto, not counting like DMS and Boruto era) and he starts the series a bit rusty right. So you have this sub Bijuu character dust off the rust to get to Bijuu level, and then as the series progresses more and more Bijuu level characters (like the Akatsuki and Sannin and different Kage) get introduced, until we make it to the war arc where nearly every relevant character is at least Bijuu level. Then you start to push beyond Bijuu level with your peak Akatsuki level peeps like Itachi and Nagato and start to bridge to Incomplete Juubi level with characters like Madara and Hashirama. Then the Juubi acts as a slope of progression by having numerous forms that get stronger and stronger, until we reach Juubito. And Juubito as well as Juubidara serve to slope and bridge to Six Paths level as they get progressively stronger, until we surpass that with Kaguya. I wouldn’t say there’s ever any cliff moments like in DBZ where characters get stronger out of necessity for the plot, there’s always a logical explanation and path of progression with Naruto.
