I think they use Coinbase Custody, so it is unlikely it is in one wallet. However, what if someone like Coinbase Custody had a flaw in their software and lost the keys to all their clients Bitcoin?
Not at all. No one would believe it. Michael Saylor's yacht sunk?
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They would go bankrupt and there would be a trial and stuff, would go down in the history books
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Setting aside that people would doubt it's real...
MS owns less than 200k Bitcoin. 200k out of 21m is maybe 1% of total supply. If you consider that out of the 21m a lot of them might be lost forever it's still probably less than 2% of total supply.
The exchangerate between Bitcoin and fiat has a volatility where 2% is just a normal tuesday. It probably wouldn't be noticeable.
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