For smartphones longevity, it all boils down to drivers, if those are not open source (and they usually are not), then phone upgradability depends on them.
That’s exactly the issue, and the bigger one. As compared to a ‘repairable’ phone. Like FairPhone et al.
Theoretically, a very first Google Pixel 1 could be upgraded software-wise with the newest Android, while the hardware can work longer. Heck, even my Google Nexus 6P and 5X could be upgraded! If not my beloved Google Nexus 4.
So, I’m not really interested in repairability (except the battery replacement, obviously) as much as in software longevity.
My iPhone 4S is still going strong, even despite numerous falls. The software though, it made it useless. I use one as a digital voice recorder sometimes, but that’s the only idea I had. It can shoot nice pictures too (in a bright daylight), and be a nice smartphone for a kid. (Because it’s very limited, and also small, and also cheap.) But the software made it useless.