If it went chest-bursting we couldn't get the scene of Wings spreading out of the armor back.
The wing make it more cinematic IMO
Amusingly, in the LN they added some extra lines for Rare's inner monologue during the previous battle royale event, where Rare lamented the players can't even damage Yorozoka-san, and she couldn't dramatically emerge from the armour like a 2nd phase boss. This implies she has been practicing on doing this exact scene since before 1st event lolFor real - she's basically a butterfly emerging from a chrysalis.
Granted, a terrifyingly destructive butterfly, but, still.