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Consider the following.
Prior to Krisis, every new wave sang their own debu wave song. You could say that Krisis didn't have their own song because they were a part of the Heroes that also did not get a debut song. But then the trend continued with TTT.
The last wave to have a traditional debut song was XSOLEIL. This song featured the singing of all the members, including Zaion LanZa. As history is aware, Zaion left Nijisanji within 3 months. Their group song is forever marred by the presence of someone that is not around anymore and was barely around long enough to matter.
What if, management did not want this situation to be recreated and instead going forward will now have wave debut theme songs created by existing tenured livers. This covers multiple things:
1. It allows the singers you have employed to sing in official songs
2. It prevents a nasty situation where a new hire leaves very early and unexpected
3. It drives up hype from long time fans (wow Enna is singing!) etc
Anyway, this is all just what I have been thinking I have no proof it's just a theory.