Bitcoin will be useless in that case. Half of the techbros wouldn't even survive the early 90's. In the 80's, forget something like properly learning Forth with Starting Forth and doing something useful.
Bitcoin already withstood a rapid withdrawal of more than half of the mining power over about a month, that time the PRC outlawed Bitcoin mining. And it also survived the relatively sudden collapse of Mt. Gox, which accounted for significantly more than half the trading at the time. And it survived its price collapsing by more than half in 24 hours, from over US$8000 to under US$4000. It seems a have pretty good survival characteristics.
In an environment where there isn't a world hegemon to run something like the post-Bretton-Woods system, international payments, if they are to happen at all, need to be settled somehow. The approach used for the 3000 years up to and including the Bretton Woods period was shipping gold and silver across oceans in boats. Before that, the Mediterranean international economy was apparently a gift economy, while intranational trade in Mesopotamia used clay bills of deposit.
In a hypothetical post-collapse future without a Singularity, there may be much less international trade. But I hope it's obvious that international trade covers a spectrum from slightly advantageous to overwhelmingly advantageous, so it is unlikely to disappear altogether. And Bitcoin has overwhelming advantages over ocean shipping of precious metals. For example, it can't be stolen in transit or lost in a shipwreck, and the latency of a payment is about half an hour rather than six weeks.
And all the blockchain requires to stay alive is about 26 kilobits per second of bisection bandwidth.
