I don't think there should be a problem with substrateless tanks.
- easier cleaning, espeically with goldfish and their large amounts of waste
- less biofiltration, substrate usually contains a lot of the nitro-bacteria part of the nitrogen cycle, not too bad as more powerful filter(s) will counter this
- less aesthetic? substrate usually make tanks look more natural, but just personal preference
I am not sure if plant growth slows with cooler temperatures. But I know that plants are highly adaptable and actually do just as well in cooler temps. I have 1 goldfish in my 10g, it's substrateless, no heater needed.
Also, you just need to select plants that don't need to be planted, like java fern, moss, floating plants, etc.
I just dump my neverending supply of duckweed, cardamine, hygrophila polysperma in there for him/her to nibble on all day. I don't plant the hygro since it's meant to be eaten lol.
Finally, I think ikuzo had a great goldfish planted tank that has substrate. I think his key was to find hard to eat plants and surrounding all his rooted plants with rocks so the goldfish can't uproot them.