First of all, i personally think iPhone 6 Won't get iOS 13 but the iPhone 6S will get it because of 2GB RAM.
In the end iPhone 5S will die in 2018
For instance, iPhone 6 can get iOS 12 but iPhone 5S won't because of 1.3GHZ similar to iPhone 5. Both 5 and 5S are 1.3Ghz... 1.4Ghz is the minimum power to run iOS 12 as it's respectable speed.
iPhone 6S even though IT IS ONLY 1.85Ghz iOS 13 recommended is 2.0Ghz and 2GB RAM. iPhone 6S on iOS 13 will be third iPhone 4S in iOS 9.
Seeing Apple is still actively selling iPhone 6S brand new, I’d assume it to be supported beyond iOS 13.