I think the point is that we really don't know at this point how long Apple decides to support a device for, and for whatever reason. Especially now that we have supposedly flagship iPhones sporting a year-old processor.
For example, I could argue that the iPhone 7 didn't get iOS 15 because Apple decided that the A10 chip could not support the new Lock Screen features. Meanwhile, the A10 iPad did get iPadOS 15 because it didn't need to. The implication is that the iPhone 7 could probably run a version of iOS 15 without said functionality, but sometimes Apple decides it's simply cleaner to cut off support entirely rather than engineer a separate version of iOS with virtually all the functionality stripped out to the point where it's essentially just an update in name only.
But I think it's probably safe to assume users will get at least 5 years of software support for their iOS devices. Anything more is a bonus.