How were the transformations pointless until the final dungeon? You still have to transform into Goron Link and Deku Link and Zora Link in the dungeons before Stone Temple.
What complexity are you EXPECTING? Because your posts don't seem realistic. "You just use Deku Link when you see a shrug nearby, you can't use Deku Link to burrow underground, eat worms and then pop on the other side of the enemy and then butt fuck him with Goron Link! That's not complex!!!"
In MM, the complexity comes from the fact you have to switch from each mask, interchangably, depending on the situation. Your argument seems completely unreasonable, and you are forgetting that in action adventure games with puzzles as hefty as the Zelda series, there's a line between complex and eye gouging.
In other Zelda games, you tend to get a unique item in a dungeon and you use that item the entire dungeon as a gimmick and end up barely using it outside the dungeon depending on the situation.
In Majora's Mask, you tend to get all of your important items OUTSIDE the dungeon, and the game preps you up and forces you to learn this item with pre-dungeon quests, so that by the time you enter that dungeon, you've already mastered that ability and the entire dungeon maximizes the game's features to create one orgy of awesome and then some.
Is it complex? That's debatable. Is it a more enjoyable way of playing a game? Yes, it is.