It would only ever be fair if Disney charged per person. Why should a solo have to pay as much as 2 adults 2 kids? How is that fair? Why should larger families, who cause more wear and tear and use more supplies, get a price cut when solos and couples don't?
A solo traveler is taking up the same space as a family of four in a hotel room, but only buying 1 ticket and 3 meals a day. A family of four is buying 4 tickets and 12 meals a day.
While Disney will have to pay more in refurb costs for hotels occupied by families, they still would be better off if they didn't allow solo travelers to book their hotel rooms at all.