IMHO, the primary reason that Holder has said he would resign as AG is personal finances.
He had stone walled the Fast and Furious search for government documents about as long as possible. It was to the point that he either surrender the required documents for face contempt of court charges immediately before the mid-term elections in November.
If he had followed that course he would lose and in losing that court battle he would lose the multi-figure future income as a partner in some Washington D.C. law firm. After all, what law firm would hire an attorney that had been cited by both Congress and the Courts for unethical behavior? It is also likely that Holder would have had to surrender his law license either permanently like Obama and Michelle or temporarily like Bill. to escape the public loss of the law license as the result of actions by the D.C. Bar.
From beginning to end Holder has done whatever was necessary for the profit of Eric Holder and nothing else.
I am pretty sure he can still be indicted for his crimes regardless of his employment status!