But a fixed ELT is no good if the hull sinks! Which is what I suspect will happen.
If PLBs are not allowed then this is a typical case of ruling in the name of safety, without actually doing and research. I'd rather ditch and have a GPS PLB attached to my lifejacket than watch the plane sink and with it the ELT. Ships have hydrostatic release systems for this reason, and if we're going to have to fit "deployable" ELTs we'll never get off the ground!