The game has several layers of difficulty (which is, for me, the ideal way to go in difficulty). The story is too easy for any player who played dozens of Mario games? It is, but it's still fun as hell. For anyone who wishes more demanding platforming levels, just wait until the post-game if you're looking for that. There are levels I really had a lot of issues to beat, even dying more than 60 times in some of them. Also, the exploration and some moon locations are quite obscure, so if someone wants a more demanding exploration... they got it too! There are several layers of difficulty to please everyone. It'd be absurd to make the game demanding from the start imho. Difficulty is subjective, I wish some people could get that now.

Also, I've just unlocked the last level of the game, and, hell, I'm really having a hard time with it.

How many times have you fallen in an endless pit or poison?

The problem is that only losing 10 coins isn't harmful at all. All coins and enemies even respawn every time you die, so you can just collect them and never really lose coins. There's no game over, so you don't restart completely the level (which shouldn't even be possible with the open world design).

The game isn't easy. There's just not enough punishment for dying.

Eh, I wouldn't say it's easy, but it's not difficult either. I think for me at least it's the perfect level of in between that the difficulty thought never crossed my mind since I've been having so much fun.