Whether or not it's classified as legal tender going forward, Bukele has indicated that he intends to continue amassing Bitcoin as a reserve asset: https://twitter.com/nayibbukele/status/1897019629702410551
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There are other ways to avoid double-spends, too, like taking the party to court.
Mining does not need to be incentivizes nearly as much as it is right now for bitcoin to still function as p2p cash.