This is needless my friend. I don't simply know where this question and comparison is even coming from, Bitcoin is decentralized, no world power would assent to that. Besides, perhaps you don't know what the "world reserve" is and what's meant to serve. If you do, then you must have known that Bitcoin can't fit into that as it has a limited supply of 21 million BTC.
The world would need far more than that for reserve.
The limited supply can't even be enough for the masses not to talk of the countries, that's even if they turn a blind eye to price stability and newness.
I understand the issues you raise here, but why do you think that Bitcoin couldn't organically grow into a role of a reserve-like currency outside the system of fiat currencies? Not that it will be declared as such, but that wouldn't even matter. It is more about what central banks hold in order to settle trades without going back and forth between different fiat currencies. Isn't it possible that at some point in the future Bitcoin turns into a global, borderless and cheap settlement layer for large transactions for central banks around the world? This is theory or call it a hypothesis, but it isn't falsified until it is falsified and not verified until it is verified.
21,000,000 * 100,000,000 = 2,100,000,000,000,000 satoshis. I think that is a number large enough to facilitate interbank settlement.
But as I said, it wouldn't be declared as such, but I believe that central banks have no legal obligations as to which currency they use to settle their transactions, do they? This is why the USD could lose its status as central banks go with the flow and if another currency seems to be more promising, they will amass that currency as well.
It is also a fallacy to believe that there can be only one reserve currency because the dollar has been titled that way for so long. There can be many and I don't see many reasons why one day Bitcoin shouldn't be one of them. Actually, it would be a great tool for interbank settlement if it grows large enough in overall market capitalization and liquidity. This is also why it could still become the best investment of today because once central banks change their stance on a currency that a growing share of people around the world start holding and using, imagine the steady growth when central banks build their reserve to not fall behind.