A gold Apple watch will not be useless in two years. It will likely be able to perform the same function that other gold watches do—show the time—as long as the battery can still be recharged. It may not last as long as a comparably priced mechanical watch, but neither will it be worthless as soon as a cell phone. (And just because many people buy a new phone every year or two, that doesn’t mean that lots of people aren’t happy to use them longer.)
It seems that the amount of gold in a gold Apple watch is likely a very small fraction of the price. I think, however, that the point that Apple products tend to hold their value well stands. They probably won’t hold their value as much as a Rolex, but I expect them to hold their value at least as well as an iPhone.