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One half of Rossy's pitch is easy, "Oh you don't like Bushiroad for such and such reasons? Come join me in my new promotion."
Rossy knows approximately how much the players make, and how much he can afford. Some players might end up earning more by going freelance, but not everybody is Unagi, and I'm guessing a bunch of players will be worse off financially by leaving.
Did Rossy actually talk yen amounts with players who are about to leave, or was it just vague promises and "trust me bro"?