A couple thoughts here.
1) I don't necessarily view the Packers as a serious SB contender this year mostly because first year head coaches basically never sin Super Bowls. So I'm just being realistic about the probabilities. But if the team is 5-2 heading up to the trade deadline, I doubt they'll think that way. With a resurgent defense and Aaron Rodgers, they will probably like their chances (and with some good reason).
2) If they traded for Sanders and he enhanced the offense and helped drive the team towards success, but success short of a Super Bowl, that would still be worth something. It's important that Green Bay gets back to some winning ways, even if it's not a championship. And it's important that they get Rodgers and the offense in a groove. They will be in better shape heading into 2020 if they have some success to build on.
3) The sense or lack thereof in making a move like this is relative to the cost. Sanders would cost them about 5M in space over the back half of the season, but the draft capital is the bigger question. Given his age and the reality that it could be a rental, a 5th would not be unreasonable, nor would it have a major impact on the future of the Packers and their ability to continue building.
4) If Sanders found success with Rodgers, he could be brought back on a one year deal as a bridge to a younger draft pick. OR he could be allowed to walk and potentially return a late comp. pick, recouping the pick traded away for him.