Well I think we've reached a plateau in terms of device specs, on most higher end devices anyways. That each OS update/upgrade has less of an impact on a device's performance as they once did, and can actually enhance their performance. In essence, a device's sustainability should be considerably longer.
Majority of my devices from the years 2017/18 rarely, if ever, leave me feeling like were lacking in the performance department, and our 2019 device with its 855 SoC/6GB RAM changes little in the way of day-to-day use. IMO, what's going to make them redundant the quickest will be the lack of support some have ceased to receive in under two years time.