He ain't retiring. The only way he is leaving the Kremlin is in a casket.
Overthrown leaders don't have a power base; mobs aren't going to turn out to return a deposed leader to power. Once they are irrelevant, they're going to stay that way, and everyone knows it.
Because propaganda has convinced every American that they know Putin personally, and understand his internal thoughts and drives. Those thoughts and drives are to cause evil everywhere, and a bottomless ego that can only be satisfied by conquering the world. It's not really a subtle caricature.
Most folks "know" far more about succession in authoritarian regimes from heroic fiction, TVTropes, and the Evil Overlord list than they do from actual history.
Mr Putin is 70 years old, and reports of his health issues are circulating. So I think that it's too late for "premature" death.
I'd be interesting to gather some data on this, but I suspect it's more common that casual perception might indicate.
One possible reason for the mysterious Russian Modified Gravity that causes people to fall to their deaths might be that Putin's position is not as secure as he might like us to think. He doesn't even need to be assassinated: once he's no longer in a position to order around force, what is he going to do? Some of the above lived for decades after being deposed.