http://www.grantland.com/blog/the-triangle/post/_/id/53167/2013-free-agent-book-reggie-bush

I think the best argument is at the end:

Finally, there's plenty of reason to believe that Bush's best years are still ahead of him. The other backs on the market — thirtysomethings like Jackson and Turner — have far more wear-and-tear on their bodies than Bush does. Having just turned 28 and with fewer than 1,000 NFL carries, Bush is almost eerily similar to one recent back who shares both a skill set and a career path with him through the age of 27:

Player A is Tiki Barber. Over the four ensuing seasons, Barber ran for 6,256 yards and scored 34 touchdowns while making the Pro Bowl three times. Like Barber, Bush has gotten more effective and efficient as his role has expanded. While Reggie's already had an extremely impressive career at the professional level, his best might still be yet to come. That's rare for a player entering free agency, and it represents a significant value opportunity for whichever franchise acquires his rights.