Conclusion first, I believe Pikmin 4 would appeal to a casual player who played previous titles before, while playing in single player mode only. If this game came out in 2023 without reputation from previous series, I doubt it will ever be popular.
I never played Pikmin before. My friend bought the game since it supports "multiplayer" so we can play "together". Well surprise, the second player is basically a nameless background support which doesn't seem to fit in the background at all. The second player can help the player by throwing items and rocks, attacking doesn't even cause aggro. It would be best implemented if the second player play as Oatchi.
I played as the second player but I'm perfectly fine with it. I mostly watched my friend playing and commenting on their thoughts. I don't think the game is balanced, it's hard to control and aim, and there are not enough ways to perform quick commands such as gather or spread-out.
I also believe the fact that it's too easy to get team-wiped makes it hard for me to recommend this game to kids.