First off, I'm wouldn't conclusively say Wells is an upgrade over PT. PT has shined when given the opportunity, and Wells would take PT's 4.8 ypc average over his first 2 years in a heartbeat. The draft is about addressing needs, and some needs hold higher priority over others. OLB need is > RB need. S need is > RB need.
With no other 1st day picks, drafting a RB, or any offensive player, is absolutely a luxury pick, and doesn't best address the most pressing needs this football team has. SP already traded away this year's 2nd for an offensive player, and to consider also using this year's 1st on another one is asinine. Yet, I pray to god Beenie Wells gets drafted before 14, because I'm afraid SP foolishly won't be able to pass on him.