Mikhail Gorbachev

History knows almost no examples of dictators voluntarily giving up or limiting their powers. Any other high-ranking party functionary elected to the post of General Secretary of the CPSU would almost certainly not have undertaken reforms that would have jeopardized his authority.

I think Putin shows that with a sufficiently determined and powerful leader, Russia can still play a weak hand competitively, and hold power for decades. So Gorbachev's decisions are critical. 

Surely it's not too hard to imagine someone holding things down for 5-10 years while Putin gained influence. And then given his track record, a very high chance Putin could have then held for 30 years.  I think 50% is not too high.