Nami has no strength related goal...as long as she is doing her job as a navigator it is fine..her fighting capabilities are just bonus....she can take care of some fodders as long as it doesn't require strong haki....but Usopp is different...he aims to become a brave warrior of the sea...so at some point Oda has to let Usopp fight some strong foes and show his mettle as a brave warrior...and at the same time, he needs to surpass his father also....Oda can't do justice to Usopp without making him strong..