Is a rigid carrier more dangerous as a projectile, or more so as an obstacle to evacuation?
In this scenario, there isn't going to be evacuation. At best, it's search and rescue. At worst, search and recovery.
The poorly attached rigid seat, with a well-secured infant, increases the infant's odds of survival, but the mass increase reduces everyone else's odds of survival (including other infants and caregivers).
There are designs and policies where it becomes "worth it." But it would have to be study to figure out at what point the tradeoff becomes worth it.
It's a grim outcome, but great safety engineering is built on the worst case taken seriously.
(Sorry, meant to reply to parent)
Scenarios without evacuations are ones where nearly everyone or everyone is already dead. That’s not a case to optimize for.