I've just been looking at some Gigabyte motherboards that are "Ultra Durable" what ever that means. And I wondered about my Uno. There are a couple of 47uF jelly capacitors near to the power jack. Being jellies, they'll dry out eventually. Perhaps even sooner for cheapo knock off boards with the cheapest of cheap capacitors. For some people the power regulator might get hot too, causing thermal stresses and eventual failure.
I wonder how long an Uno will last? Have any died on anyone from suspected old age? What's the oldest known Uno? Does anyone have an original?
I have a Duemilanove that I've used on & off since late 2010. Does that count?
By jellies, I gather you mean the aluminum electrolytics? If they fail shorted, that would be bad. If they fail open, then results depend on how dirty (ripple) the power is coming in.
Electrolytic caps dry out from high temps and internal high temps from switching currents (think DC/DC converters).
None of that on an Arduino Uno board. And those caps are not really used on USB supply.
I would say 20+ years (I have digital circuits like that in the junk box that have lasted much longer than that).
Leo..