First, good question.
Second, I'll out myself as someone who used to be in the "I want someone better!" crowd. In my case, it's because RB is "my" position. I played it, and most of my football heroes were/are RBs. Due to that, I tend to want whoever I think is the best RB to play for the Packers, always.
But, last year Grant shut me up (with assistance from some of the posters here). First, the guy, up til last week, never missed time. He was solid, he was available, and he was dependable. Second, he rarely ever fumbled or botched his blocking assignments - so he didn't contribute to negative plays - you weren't taking the good with the bad. Finally - and this part was the hardest for me to suss out - the team has no need for a Chris Johnson or an Adrian Peterson. It's a passing team with a pass-oriented offense. They want to be able to rely on the RB to consistently pick up a few yards to ensure they're in favorable down-and-distance situations, and to make sure he keeps his protections up for Rodgers. Purely a setup role, and while I'd love nothing more than 4 yards and a cloud of dust, that's just not how this team rolls.
Fans are people, and people want the best, always. As fans, a lot of us want who we consider to be the best at every position, and a place like this affords folks the chance to air out their own desires. I doubt most posters here seriously feel that the Packers should go out and acquire a Jonathan Stewart* - they just wish they would, even though guys like that are wasted in the Packers' offense.
My .02 cents.
*never happen - for one, Carolina's not giving him up.