I don't think they ever said that it would have everything upon release, but, it makes sense that they're taking this catching up approach.
Not only it allows for the new players in the NA version to experience content that is long obsolete in the JP version, it also allows for them to progressively get better at the game as harder content gets released (many of them are currently struggling with SH Loser, imagine the chaos that would have been if they released everything at once).
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