Nami's life and character don't revolve around getting the dick of someone who tried to kill her.
Nami's backstory involves losing her mother and carrying the weight of her whole village.
Sakura's backstory is about having a catfight with her friend over a shallow crush involving the class bad boy
Never grew out of this middle school level crush in 700 chapters
I don't hate Sakura as much as some do as I think she has some moments, but Kishi fucked up big time. Her character never really went anywhere.
Oda not letting Nami shine in a fight is pretty sad (not unique to her though, Usopp suffers from the same thing) , but at least the character has the excuse of not being a primary fighter while Sakura is a shinobi paired directly with Naruto and Sasuke and constantly babble about wanting to be on par with them.
Is the way Oda manage Nami perfect? No. As is the case with lot of characters not called Luffy. But comparing her to Sakura is just you being biased.
Edit :
Also lol at saying Nami is useless.
First Jinbe does not replace her, her job was never direct steering the ship. Her skills lie in her abiliy to predict and make use of Grand Line unpredictable and constantly changing weather. Jinbe himself relies on these skills of her to do his thing :
Jinbe skills and Nami skills are complementary
On top of that :
to find/reach Laugh Take one needs to translate the road poneglyh data into a chart, raw data aren't enough. We saw it in Oden flashback. Robin herself asked Nami's help to make actual use of what she got from Zou's poneglyh.
She is arguably the most vital straw hat after Robin. (I say arguably because i don't want to upset anyone lol)