Like, the real answer is Donald would have had to commission full 6 card stacks of every card and would have had to cut Harvest and, uh, what's in cornucopia, probably trade route, and like, Wishing Well, is that in there? Cut all that stuff from cornucopia just to have it. So that's why it didn't happen. 100% sure Donald wouldn't tell you it was important to his design that everyone else lose access to Trusty Steed when someone got one.
Can't tell if serious.
What I read between the lines of his design philosophies from the Secret Histories suggests to me that he wouldn't take issue with multiple players getting Diadem. I'm just guessing. If you have a different guess, maybe yours is smarter and better, that's fine. But yes, I was serious, although 100% is obviously hyperbole, but 100% certainty is always hyperbole, except for certain philosophers that think you an be really sure that you yourself exist, but those philosophers are really silly, and I'm not even 100% sure they exist.
Obviously the way Followers works, you run out of Estates if two players have them, and being the only kid on the block with a handsize attack is less novel. So maybe he would have redesigned that one in particular if he approached cornucopia and was like "the ink and paper for 30 extra cards is magically free for this set, but you can only use them on prizes." But I don't think the exclusive access on Steed, Bag of Gold, Diadem, and Princess are key to what makes the card a design that he likes, I think it's the fact you yourself cannot have more than one copy of each, and that you get them in that special way, and that they do things that kingdom cards don't usually do really.
