Honestly, with the current phone prices, cpu performances and software support, I've replaced my phones either because they became "too slow" (ie. the software got bloated) or because the battery died and the replacement cost was just not worth it anymore, especially due to labour cost of replacement.
In case of modular phones, i'd be very interested in stats of how many people buy a new replacement battery after a year or two (especially if it can be replaced with just using your nails or maybe a screwdriver, without heating stuff and losing waterproofness) compared to every other replacable component.