As soon as you take away a hard link to a real-life identifier, the sketchy people come out of the woodwork and spread images of child exploitation.
Signal has not had this problem like some platforms (e.g. Kik), and I suspect two reasons:
Lack of searchable chat rooms
Concrete link to a phone number that anyone who contacts you must know (and make it easy to identify you to authorities)
Up until now signal has been an excellent secure replacement for text messaging between parties that know each other. I hope they don’t go the “chat groups” route, though I doubt they will. But I suspect this change will make it a preferred way for abusers to exchange images and videos nearly anonymously.