Maybe we should distinguish global reserve currencies from global reserve assets. The US dollar is the global reserve currency, mostly because it's being used as a medium of exchange in international trade. Gold is a global reserve asset, because the central banks are using it to backup the value of their fiat currencies. Bitcoin can be used as a supplement of gold, and more central banks could include BTC in their reserves, but I don't believe that BTC could ever replace the USD as the global currency, mostly because the BTC blockchain can't handle a gigantic amount of everyday transactions. Gold shares many of Bitcoin's qualities. It's a geopolitically neutral asset and it isn't tied to any particular country.
USD is not only used in international trade transactions but also an asset that accounts for a significant proportion of global foreign exchange reserves. If I remember correctly, according to IMF data, the USD accounts for more than 56% of global foreign exchange reserves, down significantly from 66% in 2015. The reason is that many countries are reducing USD in foreign exchange reserves and replacing it with yuan or euro, and especially gold.
So if the multipolar world scenario happens as OP said, and if they don't want to trust a single national currency like USD. Bitcoin could replace the role of the USD in foreign exchange reserves.