Every iPhone has always gotten 5 years of support. 2017 iOS 11 does not count as it was a horrible release overall so it's more like a pause year. If 5s support is stopped on iOS 12, 6 will be stopped on iOS 13.
I can see it happening. As iPhones have increased in performance, they should gradually get longer support.
iPhone 2G - iPhone OS 2, iPhone OS 3 (2 updates)
iPhone 3G - iPhone OS 3, iOS 4 - 4.2 (2 updates, but not the whole of the 4 cycle)
iPhone 3GS - iOS 4, iOS 5, iOS 6 (3 updates)
iPhone 4 - iOS 5, iOS 6, iOS 7 (3 Updates)
iPhone 4S - iOS 6, iOS 7, iOS 8, iOS 9 (4 Updates)
iPhone 5 - iOS 7, iOS 8, iOS 9, iOS 10 (4 Updates)
Following patterns (2 generations getting the same number of updates) plus the fact that a) the 5S is very close to 6 in terms of performance and iOS 12 being a performance boost update, then I think very likely that the 5S will get iOS 12.
iPhone 5S - iOS 8, iOS 9, iOS 10, iOS 11, iOS 12 (5 updates)
iPhone 6 - iOS 9, iOS 10, iOS 11, iOS 12, iOS 13 (5 updates)
The only thing is that the 6 not a big increase over the 5S so I have no idea how well iOS 13 would run on it, and I also think, if the 6 can run it, then the 5S should be able to as well.
So far Apple hasn't given any new iPhone generation fewer updates then the previous generation (I count the 5C as the same as the 5, as it was just a repackaging). They have however given the iPad 3 (iOS 6,7,8,9) 1 less than the iPad 2 (iOS 5,6,7,8,9).