Same thing. I think that the main reason Mojang adding trident is that it's able to rip through the water.
So yeah, prismarine arrows would be pretty useless while having a trident.
I bet they're going to add all 4 of them. At least I hope so.
Maybe, they said that they're going to add only one of them to start a hype train?
As we can see at this moment, each new mob expands game by a little. They spawn in different areas and have their own interesting, unique, never seen before features. If we combine all of them, that would be MASSIVE update!
I hope that Mojang is going to say at Minecon Earth that it was a joke and they have planned to add all of these mobs from the very beginning. It would be like if it was an update that expands everything in Minecraft by a little.