Rev 17:10-11 And there are seven kings: five are fallen, and one is, and the other is not yet come; and when he cometh, he must continue a short space. And the beast that was, and is not, even he is the eighth, and is of the seven, and goeth into perdition.
I am going to throw in a thought.
And the beast that was, and is not, even he is the eighth, and is of the seven,
I wonder whether the leaders/kings are actual men, not kingdoms. The reason being the beast, or man of sin, was a member of the seven, and he was a man.
The following text then may refer not to the time of John, but the time of the beast, or man of sin.
five are fallen, and one is, and the other is not yet come; and when he cometh, he must continue a short space
Five leaders may have passed (not necessarily died, but moved on), and left their position. The beast could be any number of them, but not the seventh, who is yet to come.
The vision could be showing the state of the kingdom at the time of the beast, not the time of John. The kingdom may have only had six leaders when the beast is revealed, not yet seven.