>What do you think would our society today look like, if more elecronic products like smartphones were designed with a much longer lifetime in mind? Would the use-value of the products stay the same over time, or would it decrease with technical progress?
You get more standardisation, less form over function sacrifices (works better but has worse design), as far as durability goes , it depends long lasting batteries are just hard to make, what you definitely would get is better repairability.
For smartphones you'd probably would have seen magnetic field tracked pens (used in graphic tablets) rather then the currently capacitive touchscreens, because the magnetic pens are easier to make and more precise, Resulting in a renaissance of handwritten text and social/cultural trends/development around penmanship and quick sketches.
You'd probably would have seen a universal interface scheme, where all sorts of devices no longer would have buttons and nobs but rather an IR-port that turns your smartphone into a remote for everything. Also a much greater R&D focus on sensors turning smartphones in a portable scanner.
Since socialism has a sort of protected commons that isn't at risk of being enclosed, you'd see more people contributing to projects like user-generated "cartography" type stuff where people digitize "note-worthy" features of "meat-space". Enabled by the scanner functionality.
>Would the use-value of the products stay the same over time, or would it decrease with technical progress?
I'm not sure what you mean with this, you might get better backporting for software to run on older technology, because there aren't any IP laws blocking people from pruning software to make it run.