By this logic, gold and silver rounds are "unconstitutional."
The original coinage act specified that the government was to make mints available for people to coin currency to a standard. If you were to bring silver or gold into the mint, it could be coined as legal tender valid for satisfying debts in the eyes of the law.
Crypto is no different than an asset. It is what it is. I do not believe it will replace the dollar, but I do believe it will play a critical role in the world moving forward. I don't think the concept is fully matured or "battle tested" just yet - and any idiot can claim to have a crypto-currency for people to throw money at - so there's a social maturity that has to be reached with it, as well - the same way we had to stop believing in nigerian royalty sending us emails. Email and the internet weren't bad ideas - just thinking you were going to get rich by sending money to po boxes.