Can you explain once and for all why a "Curse - X" dualtype is a bad idea? Or if you think it CAN be done, what needs to be done to make it possible? Some extra setup instructions perhaps?
Since Curse is both a card name and a type, it would be confusing to have any more cards with the Curse type. It would create the question, what are Witch etc. referring to? Can I discard this to Mountebank? And even if you say "well it only makes sense that they did this if Witch can dish these out," whatever, yuck, it's awful. If I wanted more Curses, I had to make the type and name of Curse different back when.
Then consider the case where I want to add a new Curse so badly that I do it some other way. There's a Super Witch and it says (after a dividing line) "In games using this, when a player would gain a Curse, they instead gain a Super Curse." Let's say Super Curse is -2 VP. It is not a "Curse" so no problems there. It's clear whether or not you can discard one to Mountebank.
We have Super Witch and Witch in the same game. Well why buy Super Witch? Super Witch is balanced around Super Curse and Witch isn't. Witch is way better at dealing out Super Curses.
If Super Curse were comparable to Curse - just about as bad to get, no better no worse - then Super Witch wouldn't need to be weaker than Witch, and I could buy either card depending on other factors (you could also let the player choose which Curse they took, which makes all Cursing cards weaker but if it's not by much then why not). It is far from trivial to make Super Curse comparable to Curse though (aside from making it identical), and the less it matters which you get, the less exciting it is to do Super Curse in the first place.
Even if this all worked out, it wouldn't scale unless you got 50 Super Curses. What kind of expansion would have room for that many non-kingdom cards?
Finally there is Dark Ages. I went for it, I put in 50 Ruins, They are not as bad for you as Curses but the cards that give them out are balanced around that, with the existing cards that give out Curses still just giving out Curses in those games (albeit, Curses that hurt more because now you can get 20 dead cards, not considering Moat etc.). Dark Ages had 500 cards and it seemed like I could make room for Ruins. If there are more sets in the future, they won't have that space and anyway I did it already, it would be way less exciting the second time.
How did you arrive at the "1 Action, 1 Buy" principle?
Playing one action per turn is extremely simple and opens the door for making cards like Village and Spy (and less obviously, Remodel and Vault and Bank and Gardens). I value both of those things. I made a TCG that had you just play one action per turn, as part of an attempt to make an extremely mainstream TCG, and it worked great. So I already knew it was a fine direction to go in. I wanted something simple and went for it. It immediately worked well so that was that.
In my initial notes it was going to be that some cards let you buy cards, but that seemed bad once I thought about it. It had to be that you could just buy stuff. I didn't have Gardens etc. at that point and could have just let you buy multiple cards, but again I knew that limiting you to one card was simple and would let me make Market. Village and Market were maybe the 2nd and 3rd action cards I made; they were inherent in the game premise.