I have found one duck on my transatlantic and I took it with me (Solo traveller, no kids), I enjoyed finding it. Especially because it's a tuxedo duck and it was formal night.
I think it's adults who decide to bring ducks on cruises to hide for others, I do not believe it's kids. Usually the parents are in charge of packing and thinking about baggage allowance. Kids might get more fun from it IN THE MOMENT, Adults before and after.
I also think it is something that can get out of hand due to social media. To me it's on the same level as Fish Extender gifts. For some it's bringing something small & cheap, others spend hundreds of dollars on it. And I think that it can fool first-time-cruisers who think this is what they need to do to enjoy their cruise.
Either way, if it's something that prevents crew members to do their job properly or it's a safety thing, it has to go.