Aren't Apple OS updates typically tied to actual hardware constraints (vs a x-year obsolescence cycle)? With the 5 and iOS10, Apple stopped supported 32-bit hardware, right? I don't recall a time where Apple just said "that phone is too old and slow" - it's always been "that phone doesn't have [hardware architecture X]".

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.