You won’t be able to buy Bitcoin with digital currencies. You may still be able to use cash (so long as that’s around), or if you’re lucky you can find someone who takes gold or silver.
Most likely you’ll have to sell either goods or services to acquire Bitcoin in this future state.
This is in fact very good for Bitcoin - every transaction that people want to keep away from the eyes of the State will be on Bitcoin so demand is gonna go way up from where it is today. Sure it’s not global reserve currency, but it’s global money for the blackmarket and the blackmarket could extend to things like tradies wanting to do the equivalent of cash jobs today, meat if they want to introduce carbon credits, etc.