@Lustig Andrei,
I dont think Hagel was up to the job but being Obama's Sec Def is a super sucky job. You have political idiots in the White House thinking they know more than then military men, increasing threats, degreasing budgets.
I think Hagel leaving all boils down to money, in the last two months Obama made it clear to Hagel that he expects the Defense department to do a list of things, and he told Hagal how much money he would have to accomplish these things. The two dont match, the expectations are unreasonable.
A new guy in the Sec Def chair is not going to solve that problem. The problem is Obama's lack of hold on reality.