You kind of answered your own question there. If anything collapsed the power grid, bitcoin would be worthless. Precious metals would be the go-to asset, with guns and food. But would a war cause that worldwide? Probably not.
Personally I believe that bitcoin is going to exceed gold and silver simply because you’re not going to be bothered to cut up a piece of gold, verifying its purity, and weighing it to the precision of hundredth of a gram just to be able to buy a loaf of bread. It’s way too much time and if you are only 0.01 grams off from the actual figure you could be losing a shit ton of money after a few trades.
Bitcoin on the other hand is easily accessible by anyone, easily transported, and easily transmitted to anyone all over the world.
Unlike people above me suggest, it is very unlikely that the electricity grid will go out simultaneously all over the world. Also you have to realize that electricity grid is much more reliable than what was in WWII and WWI. So bitcoin will still be able to be used.