It doesn't eat the lunch of everything. It's seen as a store of value and a portfolio hedge by the majority of institutional money that gets involved. It's simply going to eat the lunch of those two categories, and in an investment portfolio gold is the most likely to be impacted.
Also...
What good is Bitcoin if you run out of food/water?
In your dystopian future, what good is gold if you run out of food and water? Gold has been a terrible investment for the last several decades and will be even worse in the coming decades. If the dollar continues to be debased you want to own literally anything priced in dollars. Stocks and real estate (among others) will all do much better than gold. You could throw a dart and hit something that will outperform gold over the coming decades.
Gold is only valued where it is because people have believed that it should be. There really isn't much special about gold. Once people stop believe that, mainly because they now have a better alternative, the price will move closer towards it's industrial value, which is significantly lower than it is today.
I see literally almost zero reason to own the slowest horse in the race.
Right... exactly. So decentralized (but not necessarily efficient) systems need to come into play, and that includes physical currency. Including food production, water collection & purification, construction, and other means of production.
There is a reason why SWAT teams have raided citizens at gunpoint for collecting water.
I'm not going to speculate exactly how that all goes down but it's pretty weird.
The entire system needs to change from the ground up.