Barkley would have been the second or third QB taken this year. If he has a great senior season, he could be the first overall. That would come with much more money. Kuechly is expected to be a mid-first rounder. But he is unlikely to move much higher. Teams just don't use No. 1 overalls on linebackers. With new slotting of draft spots the signing bonus between No 16 this year vs No 9 next year is nominal. The difference for Barkley between No. 10 this year and No. 1 next year is significant.
Barkley has the chance to be part of a national championship. That sort of team accomplishment is worth delaying a professional career. Most never get a shot at a championship. Kuechly won't be playing for a national title. He is better off getting his pro career underway rather than risking injury in the Tire Bowl.
The real unknown is Luke Kuechly's elbow injury. If he cannot work out at the combine then returning makes sense as he would need the senior season to prove he is healthy.