You are commiting an etymological fallacy. What counts first of all is what legal tender has been in name when the constitution was written. Second, the meaning of the word when the bill was written may have reasonably changed and it may still be sensible. If this is warranted, because the state has the power to mint gold coins, they can peg the gold coin to the bitcoin, if you want to see it that way.
This is illusory of course and there might be regulations in place to prohibit this, and there might be no other ways around it.
Still, this isn't simply simple and you are selling it short.
The states do not have the power to mint gold coins; they only have the right to require that gold coins be accepted for the payment of monetary debt. Only the Federal government has the right to mint coins, a power reserved by Article 1, Section 8, Clause 5 - which similarly reserves the right to determine value of coins.
This is very simple, it is very straightforward; anyone with a grounding in the common law would understand it exceedingly clearly - doesn't the fact that everyone in this thread disagrees with your interpretation at essentially every level give you a bit of pause?
This is illusory of course and there might be regulations in place to prohibit this, and there might be no other ways around it.
Still, this isn't simply simple and you are selling it short.