Misogynist wanker class system! Pretty much. This was brought home to me again in reading Anne Glenconner's book. She was the eldest child of the Earl of Leicester but because he had no sons, the title and her childhood home went to a cousin.
On the question of whether the royal dukedoms will revert to being just regular dukedoms after they pass out of the hands of HRHs, I think the answer to that is yes, but in the absence of male heirs down the road they would revert to the crown and so could become royal again.
Princess Mary was Countess of Harewood by marriage, not in her own right. Princess Alice, Dowager Duchess of Gloucester was allowed the style Princess Alice although she was not a blood princess. The Queen granted her request to use that style but did not create her a princess of the United Kingdom.