Yes, I know one is from different eras and the other is not. However, one is based in real life and the other is not. There’s no reason why Nintendo couldn’t just invent a reason why, say, all the old maps got a redesign, or why your character’s wearing a new style. Literally none. It’s a fictional, almost-absurdist universe where anything can, and has, happened. It can choose whichever aesthetic it wanted, or change whichever things it wanted, and, as long as it kept the core identity, it would still feel like Splatoon.