5 had already fallen, and one was in the process in John's time. There is no such thing as these Kings falling in the future.
Also, 10 horns means 10 rulers/kings, with the Ac as the ruler above them all. There is never only 7 rulers anywhere in Rev.
This is difficult for me to explain, and it took me many years to get to this place. It may or may not be true, but just FYI this is how I see it.
The entire overall emphasis in the book of Revelation is on the endtime development of the 4th Beast of Dan 7. In the endtime scenario there are 10 horns, who are 10 kingdoms, and 3 of them are defeated, leaving 7 leaders and 10 nations.
And that's just what we have in the book of Revelation, the Beast Empire, centered in Europe/Roman tradition, that has been dividing for some time into independent states. Ultimately, a coalition of 10 will form, and the Antichrist will appear to assume control over them, defeating 3 national leaders. This is how we know it is the Antichrist, in my view.
So in Rev 17, the Harlot is the city of Rome, who has, like ancient Israel, turned away from Christianity to modern paganism. She rides upon the Beast system, or the European confederation of 10 nations. The Antichrist assumes control over this Empire, and at some point burns the city of Rome.
John explains this Beast of 10 horns and 7 heads by cryptically showing Christians that he is indeed referring to the Roman Empire. He dare not come out and say it, because that would appear to be a form of rebellion or sedition. So God leads John to depict "Mystery Babylon" through the use of two figures. The 7 heads of the Beast, John says, present a figure of seven historical kingdoms, the 6th of which is ruling in his own time. The Roman Empire was ruling in John's time, so John was identifying Rome to Christians without coming out and saying it.
The 2nd figure John uses to show the 7 heads represent the Roman Empire is by using the figure of seven, once again, to depict 7 hills. Again, 7 hills were associated with the 7 hills of Rome, which the Christians would've known.
The Harlot is called "Mystery Babylon," indicating it is a cryptic reference. Babylon was the 1st of the 4 Kingdoms that would lead to the Roman Empire. So "Mystery Babylon" is a cryptic reference to the Roman Empire, in my view. And it is clearly identified at the end of the chapter as a city that rules everywhere in that part of the world. It had to have been Rome that John was referring to.