The biggest problem with longevity of modern portable electronics (watches, phones, etc) is that they usually use the internet. That generally means they need security updates when bugs are found, protocol updates as the net evolves, and sometimes they depend on specific network services that the provider drops.
Sometimes the electronics themselves fail. For example I've got a Casio MG-510 guitar [1] that I bought in the late '80s. It still works great as an electric guitar, but its MIDI functionality no longer works.
From what I've read this is almost certainly due to a bunch of bad electrolytic capacitors. Someday I'll have a go at replacing the capacitors, which other people have reported fixed theirs.
[1] https://spinditty.com/instruments-gear/The-Casio-Midi-Guitar...