Game play and story segregation involves "inconsistenc[ies] in how things work or behave between the gameplay [sic] and storyline sections of a video game, the latter of which generally consists primarily of cut-scenes and dialogue." (TV Tropes) I fail to see how the story, which involves Rosalina needing Mario's help contradicts itself. The developers clearly intended for Rosalina to need Mario's help. This is the story, not game play. The cut-scenes and textual data proves this.
If you go by Galaxy alone, it's just straight up plot holes - in cutscenes, Rosalina shows abilities that more than enable her to take on Bowser, despite her requesting help.
But now, with 3D World, Rosalina has been shown taking on multiple powered up Bowsers at once:
Even if you continue to be stubborn and ignore the obvious plot hole in Rosalina "needing" Mario's help in Galaxy and decide to ignore the abilities she has, currently, she has been shown defeating him in a main series Mario game with a limited arsenal (and one that is IDENTICAL to what Mario had in Galaxy - save for one or two extra abilities, a BETTER spin attack than what Mario had, and the omission of Star Bits (which were useless against Bowser in Galaxy anyway). So she can take Bowser just fine. That much is certain.
Also, Miyamoto has said in the Iwata Asks regarding 3D World, that Rosalina is made to be "a regular girl" in Mario Kart specifically, and such is why she also has a limited arsenal in 3D World (as her god-like powers are completely unnecessary in these family party games):
"When Super Mario Galaxy first came out, I was a little hung up on why Rosalina was showing up when Princess Peach was already there. I thought it was fine as long as it was only Super Mario Galaxy, and we made her design so she's a little bigger. But all of a sudden she was a regular character in Mario Kart as just a regular girl, even her size turned to normal."
http://iwataasks.nintendo.com/interviews/#/wiiu/super-mario-3d-world/0/3
They' sacrificed themselves. You see this when Mario's luma leaves Mario and it plays that tear-jerking theme. We see Rosalina's observatory also sucked into the supermassive black hole. In order to not die, Rosalina would have needed to leave the Universe, since everything in the Universe would include her.
The Luma merely said good-bye to Mario as it was accepting the inevitable (the end of this universe, and it's death in this universe), and it was never stated to be anything more than that. The tear-jerking theme plays because it is actually Luma's theme in general, according to the soundtrack, and according to Smash Bros too (it's incorporated into the remix of "Rosalina in the Observatory").
The logic behind this "Luma's sacrificed themselves to create a new universe" point is riding on "Rosalina said they can be reborn as galaxies when they die", but the keyword there is "can". They are not always.
Your assumption that Young Master Luma waves goodbye to Mario because it is going to be reborn as a Galaxy to form a new universe is incorrect, as Young Master Luma does not get reborn as a Galaxy - it is merely re-incarnated as itself, only with memory loss. Meaning it was not sacrificing itself.
All of the other Lumas were just being sucked in like it.
In order to not die, Rosalina would have needed to leave the Universe
And she feasibly can - she's got a universe beneath her gown.
Read the post of mine that has been copy-pasted all throughout this topic, it's explained there.
Rosalina seems like a deity, but nothing is explicitly stated. I didn't see Rosalina reset the Universe.
You did, but for some strange reason, you tried twisting it in your head to make it seem otherwise.