Apple is generally about two things: incremental improvement and the complete package. This lets them diversify the yearly 'wow' upgrades across many components

Apple is generally about two things: incremental improvement and the complete package. This lets them diversify the yearly 'wow' upgrades across many components: cpu, graphics, camera, display, IO, battery, chassis, etc, based on whichever step changes reach maturity that year. For instance, this year is not only the year of the M1 but also the Mini-LED ('XDR') displays we see on the new iPad Pro. 1000nits (peak 1600) is a massive improvement as is the improved contrast. Meanwhile users are on their own personal replacement cycle (say 2-4 years for phone, 3-6 years for iPad and Mac). So by the time the user is thinking about replacing their device, there's several years of accrued changes across many/most components that make the latest device really compelling. You're right that at this point all the Apple devices have at least a good 5-6 years of 'works fine' in them. But this fall when you go to look at a new MacBook Pro and it's got an XDR screen and an M2 chip that together have you 3-5x screen brightness (vs your 2015's 300nits), 3-5x graphics, 2-3x cpu, 2-3x battery life, better webcam, etc, all compared to your slowly wearing out macbook... that'd look pretty darn enticing to anyone. As for the future? At some point component capabilities (like cpu/gpu perf per watt) reaches a point when entire new categories are unlocked, like AR/VR or computational photography effects, but those have to be incremented to as the pace of component progress allows.