> Why do phones become obsolete so quickly when other computers can be perfectly usable for well over 10 years?

I'll answer as if this is a real question: the overall rate and pervasiveness of change for that class of device.

I wonder how well a nearly 10 year old smartphone would work with modern software? Has anyone tried something like a Note 3 with updated software? It might not be terrible, tbh.

I recently had to use backup phone and used OnePlus One. Updated Lineage OS and it was… fine? The biggest problem for me has been battery life. I even tried to buy new battery but its hard to find ones that are not a bad scam.

It makes sense. The Snapdragon 800 and 801 weren't particularly slow. While cameras have certainly improved, even those weren't bad back then.

I suspect that 2022/2023 flagships would hold up really well 10 years from now.

I mean, you are talking about a general purpose computer + quite great camera + multimedia device that fits in your pocket and is battery powered. The only ones we get close to 10 years of review were pretty much experimental phones, only recently did we reach a much more slower pace of development where a current gen phone is not that different from a 3 years older gen one. Current ones use much more durable materials (steel frames, glass which is contrary to “breaking easily” is one of the hardest materials, thus being very scratch resistant), and better, more performant hardware that won’t become multiple times better year-on-again, so besides replacing the battery every 3-5 years, they might very well have a longer lifespan.