My answer to this is very simple actually, if there is no business case to go to Mars I don't want any government stealing money from people to go to Mars because at that point it is all it is: theft.
Eventually a business case for Mars may become real and then businesses will find a way to get there. Today it is likely not the case at all that there is any sort of ROI on going to Mars except for raising spirits of those, who want to see it happen.
Well, if the people who WANT to see it happen actually PAY for it by BUYING bonds that would pay for it, then a private business can do it without government! That's because a private business can print bonds that can be sold (tentatively) to people and if enough money is raised then actually collect the money and start building.
To do it otherwise is to steal, but that's nothing new, that's what all governments always do.