Your statement assumes uniformity between arts regarding what a 'dan' is. But while my sympathies lie with your sentiment, intellecutally, I cannot agree with this.
If a person creates their own system, determines the ranking system, and is the highest ranking person in the system, then they have every right to call themselves by that rank. Just as anyone who owns a business can call themselves the owner, CEO, or president and actually
be the owner, CEO, or president.
If you develop your own system and get ranking through an organization, such as the
World Blackbelt Bureau, people call you a fraud. If you develop your own system and are awarded your rank by your students, people call you a fraud. If you develop your own system and award yourself your rank, people a fraud. If you get people in other arts to award you your rank, people call you a fraud.
So no matter what you do, people call you a fraud. So long as you don't make a big point of going around questioning everyone else's legitimacy, I don't really care how you get your grade. Your system and your ability will be readily apparent, and you and your system will either fade away inside of a century or you will establish a lasting legacy. History will judge you, but I have better things to do in the mean time.