The fact that the 5s got iOS 12 is more of an exception than the rule. The 5 stopped at iOS 10, and the 4s stopped at iOS 9, so the general trend should have stopped the 5s at 11.
Point being, the cycle isn't known. The iPhone 7 could be supported for many years. Or, hardware design could change in 2019 and even the XS and XR could be obsolete.