I think people should remind the situation when Putin took over. Russia had gone through a "time of troubles": loss of territory and influence, civil war, an incompetent leader taking power in a coup, collapse of the state bureaucracy and power grabs by oligarchs in the leader's court... Putin then was picked to take over as an "outsider", a supposed technocratic temporary stand-in. What are commentators saying about this new PM?
His goal then was to stop and then reverse the devastation wrought by Yeltsin. He succeeded way beyond what anyone expected. I guess that now his goal is to prevent the waste of all that effort, a new time of troubles starting. Perhaps he does want to find "another Putin" as in a outsider who was not full out to get power. Or has already found one.
Also, I think that people like him operate on a level where their corruption is rather irrelevant. The concern is one's work, the legacy in history. After more than 20 years he didn't play any short-term role, to retire into some pile of loot. I remember some of the silly smear attempts against Fidel Castro: "he had a fortune stashed away"... of course not, he was not playing that small-minded game.
Though seeing this only from afar (not a russian, can't be providing answers on this tread, only a comment), I don't see Putin simply retiring. But neither do I see him doing anything that might endanger the stability of his country, his legacy, once his term is over. My guess is that he will leave power, and this new PM may just turn out to become the next president. In a role less imperial, finally unmaking one of Yeltsin's remaining disastrous actions.