I don't know much about the internal workings of the Russian government, but how loyal is the FSB (which succeeded the KGB, NKVD, etc.) to Putin? They could probably prolong his rule at gunpoint for a while. And can Putin - alone - launch a nuclear strike?
I remember there was some internal unrest when the Soviet Union fell and Boris Yeltsin got in - if I recall, the Red Army didn't do as they were told and smash the Russian people to protect the old order. The old KGB thug running things now probably purged everyone he felt he couldn't personally rely on.