It’s a private venture. I don’t think they have to prove anything to anyone.
As a private venture I certainly agree they can do what they wish to with their money and time, and if it is a go, I will follow with interest, and hope for their success.
However, as a private venture, I suspect no one party will be able to foot the bill, and that they will look to investors or donors. Investors would be looking for some type of return on their money. Donors, not so much.
So, what they would have to “prove” largely depends on where the money is coming from. No bucks, no Buck Rogers... (Shamelessly stolen from Apollo 13, where it was probably stolen from somewhere else)