Although Monet's Astronomy Knowledge might come to play in the future Space Arcs.... It cannot now since she is dead.... I looked up those Chapters again and her death is shown....
It's not
clearly shown. The most logical assumption is that the way Oda portrayed that scene was done in a way which would make his viewers think more than twice about what it means; and it's simply two-way illustration. Since it shows a stabbed heart and Monet coughing blood, leading to her saying something about Doffy. No death is shown. And since this
is Oda's work, and since this
is One Piece, and a real death in OP is when we
actually see them die - ala WB/Ace (unless it's a character who can provide no relevance/minor relevance to the plot). Pell is a good example of this: he survived a 2.5km nuke and him living brings no meaning to the story. Just as Monet dying brings no meaning to the story. "Loyal to Doffy." Sure, that means something, but Oda could've done this through anyone. But he didn't, meaning he has bigger plans set for her. However, her living, has such great relevance to the plot.
Robin was not loyal to Croco which was evident from her first appearance as she tried to directly help Straw Hats.... Whereas Monet was loyal to Doffy enoungh to throw away her life for him....
We never saw Robin's past until she became a true nakama. What if Doffy saved Monet as a child, and since then Monet has been loyal to him. And remember, Luffy has the "most terrifying power in all the seas" (turning an enemy into a comrade). What's to say Monet is alive and has plotted the demise of the SH's (with Joker the only one knowing) but along the way there's an incident with Luffy or something like that (Oda can obviously do it).
Franky gave Usopp a hard time.... But normally pirates are bad themselves.... Straw Hats are one of few exceptions.... And Franky clearly tells Usopp that SH's being pirates might have stolen the money from someone and now he was taking it from them in turn....
And pirates are also loyal to their captain. But - times - they still think they should be something bigger: Zoro giving his life for his captain, but after his training, he wanted Luffy to be prove he is stronger than him, otherwise he would've wanted to become captain. And it's obvious Luffy wouldn't let that happen, meaning Zoro would've defected from the crew or killed him.
Oda can easily make all this add up and make sense, hence why I believe Monet becoming their nakama is a better bet in comparison to Rebecca.
