That's a pretty hefty accusation coming from the person who missed the point of my comparison.
At the risk of sounding more flippant than I intend to be: I didn't know how to respond because I don't think you grasp how story structure works. I don't think you going off on some tangent about emotional subtext addresses what I'm saying either.
Let me try this again:
Training montages are a common thing in fiction, and don't automatically preclude main characters from being main characters. The hero isn't constantly going to be involved in the central conflict. Sometimes the hero is either absent from that conflict or in the process of preparing for that conflict, often to facilitate a sense of payoff or development later on.
This applies to Goku, who develops a personal connection with Merus (another main character in the story and supplementary mirror to Moro, who has a stronger connection to Vegeta), trains with Merus to grow in strength while developing an ability that specifically required him to think about the principles of Super Saiyan and go in the opposite direction, and presents the results of his training to an antagonist his previously equally powerful friends struggled against.
This applies to Vegeta, who is given the bulk of character development in the arc and thoughtfully prepares a counter to Moro's abilities. He may very well have more story significance than Goku - that's yet to be determined - but you can have multiple main characters in a specific narrative. You can make these characters more important than the supporting cast by A.) establishing their prominence early on while foreshadowing their roles, and B.) following their journies throughout the story while keeping track of their progress, as this arc plainly does with Goku, Vegeta, Merus, and Moro.
Suggesting that important characters are as insignificant as secondary ones when absent from an ongoing conflict doesn't make one iota of sense, because all four of the main characters have been absent at various times. All four of them are still vital to the plot's progression and themes. All four of them have pushed the story forward, especially prior to the invasion of Earth. All four of them have developed or bonded with another in some significant way.
All four of these characters have carried the story: Vegeta more than Goku at some points, and Goku more than Vegeta at others - particularly right now. To imply even one of them isn't deserving of the limelight is incredibly shortsighted and absurd.