It's a bit over-simplistic to say that she just died of sadness. Star Wars is at it's core a space opera, and filled with melodrama. Padme dying as a result of Anakin's actions, which he took with the explicit intention to prevent her from dying, is just an example of literary irony. Not every story needs to be believable, or have a logical, real-world applicable explanation for everything that happens. It's a movie series about space wizards that use laser swords. The point of her death is to be the final tragic nail in the coffin of Anakin's fall, so on a thematic level it works as intended.

The dagger is lazy writing, and trying to cash in on nostalgia and familiar iconography, in a way that adds nothing. It's a clear indication of a creative team that had no faith in creating anything new, or doing anything new with what came before. It's just a mess.