Because most other characters haven't nosedived the way she has. Nami is the storytelling equivalent of a washed-up rock star. All her best "hits" happened years ago, she hasn't done shit since, she's hit a downward spiral, yet we're supposed to believe she's still good now?! No, she was good then. Now she's terrible.
So to use your example, then that makes all her past hits worth nothing? Some people are all about "what have you done for me lately", but others are attached to their fond memories of those old hits and hope for more in the future, even if it might be unlikely. No need to call their entire career a waste.
Except the Arlong arc went out of its way to portray Nami's greed as having a noble cause. ALL of her money hoarding over the years, even as a child, went to paying off Arlong. At that point there was a sympathetic reasoning behind a trait that made her friggin' betray the crew. Some level of greed and thievery I could accept, but when she started haggling Zoro for several hundred percent interest on money he needed to buy the swords he needs to friggin' fight, I was taken aback. The fuck was the point of that, other than to be a complete and total bitch?
And when did we see her really busting his chops over all the money he supposedly owes her? She used it once to get him to go save Vivi at the end of Whiskey Peak, but that was more to actually get him to do something. I don't recall seeing her actually going all loan shark on someone and demanding her interest money. I always considered it more of a teasing thing that maybe she's keeping track of and maybe she isn't, but regardless, she's not being a hardass about it.
And that was back when she still had worth as a character. She lost her worthwhile traits, but kept the bitchy qualities, and gained a shitton of exploittation. Again: faaaaantastic.
Personal interpretation. I won't deny she's taken it in the shorts in terms of fanservice and important things to do lately, but my frustration with that comes from thinking her character deserves better.