According to a data analysis based on public filings, crypto firm 21.co estimated that the U.S. government still holds 194,188 BTC, estimated to be worth $5.3 billion. The firm noted in its analysis that these are “lower-bound estimations of the U.S. government holdings based on publicly available information.”
https://www.binance.com/en/feed/post/1198419
That would be a remarkable turn of events if it were to happen worldwide, where FIAT currency collapses, and Bitcoin reigns supreme. It's not impossible, but it's unlikely because no country would willingly adopt something they can't control. We're well aware of how corrupt some nations can be, even if they may appear progressive, so I have my doubts.
As for the Bitcoin they are holding, it does seem a bit odd to hold onto it since it was confiscated from illegal activities. In a typical scenario, it should be destroyed, but unlike illegal drugs and weapons that can be disposed of, Bitcoin is a digital asset. Therefore, they've chosen to monetize it and consider it as income. Since they've publicized the amount of Bitcoin they've confiscated, the public will indeed be keeping an eye on it, and they might not have many options other than selling it to exchanges.