Kids got a broken arm or a broken nose from being slapped around, abuser can't hide it forever. Eventually kids gotta go to school or cops are going to do a welfare check. Same thing if kid disappears off the radar.
SBS is not just a parent trying to shush a baby - it requires a very significant amount of force in a healthy infant. This is not a "oh shush let me hug you" thing, this requires repeatedly seriously jostling the baby hard enough to pinch it's airway off or bounce it's brain off the inside of it's skull. Usually repeatedly.
And as a general rule, KILLING people because they made a mistake is really fucking dumb.