1) Does that mean that even if bitcoin was to completely replace gold as a store of value, the realistic market cap would only be $5 trillion at the current time?
Realistic marketcap is the weird definition I've ever heard of.
Bitcoin has a public ledger and public whitepaper. You can know what is circulating supply and total supply of Bitcoin. With temporary price of Bitcoin and circulating supply, you can get temp marketcap of Bitcoin.
If you don't want to calculate, check it on
Coingecko. You don't know the lost bitcoin so can not get actual circulating supply. You can not get lost or actual marketcap, and you only have marketcap.
2) The above assumes that central banks would not hold any bitcoin. Is there any rationale for central banks holding bitcoin?
Bitcoin is for everyone on the planet. Banks can have Bitcoin, they have rights to buy and hodl Bitcoin. If they hodl Bitcoin, the game will have big participants and manipulations will be more serious.
Bitcoin is a speculative asset.