Goldfish 2 was a monumental upgrade. Now I hate to say it, but at least for Prolific Publishing, their goal is to sell copies. More development is only needed if sales numbers start to slip or a competitor comes along.
This is not to say that Eric Daniels won't personally find a reason to upgrade the Goldfish Aquarium. Jim and Eric have a love for their respective projects and won't let them languish for too long.