I'd say that it's tough to make a definitive statement on the maximum lifespan of phones.

For one, Fairphone is selling phones with 5 year warranties, so I don't think there's any argument that a phone with a replaceable battery and continuous updates shouldn't last at least 5 years.

With regard to cellular technologies, I think it's hard to compare the technologies of the 2000's with the technologies of the 2010's and 2020's. Smartphones radically changed the purpose of cell networks, which meant there was a rapid shift in technologies in the 2000's.

That said, 2G networks like GSM and GPRS are still around and are only set to be shut off in some countries around 2025. GSM is from 1991, and GPRS was standardized in 2000, but the protocol existed as early as 1993. That's 20-35 years that you could use your old StarTac or RAZR, though, those are not smartphones.

3G didn't last as long as 2G did, as it was more of an overclocked 2G developed as a bandaid to keep up with smartphones, as opposed to the packet-switched network that we have now with 4G and 5G.

Looking at the most recent technologies, LTE and Evolved Packet Core are still the backbone of our cellular networks, new wireless standards like 5G NR just piggy-back off of existing 4G networks. LTE has been commercially available since 2010, so again, that's over 13 years of radio compatiblility, with likely another decade to go.

Considering how much smartphones have matured and how the pace of releases have slowed down, I don't think it's unreasonable for the average smartphone to last 10 years like laptops do. It just requires better compatibility and maintenance on the software front, which Google has already been improving on for the past 5 years.