General agreement is not how it works. Consensus is. General consensus is used in open source projects for code, and even in Bitcoin, but that’s just code. To get people to run the code you need support from all parties.
I understand that Ideal Money does not touch the concept of medium of exchange, which is why it suggests for government money to be based on Ideal Money. If Ideal Money is also a very good medium of exchange, there is no reason for government currencies to exist. I don’t think Nash had any objections to this idea.