You win some, you lose some. On the one hand, Angelique would have lost all hope forever of obtaining Barnabas if Jeremiah had killed him in the duel, so she came out OK. (I wish I knew more about that medallion.) On the other hand, since Barnabas has killed Jeremiah, Josette is now free to marry Barnabas, and Angelique may have to do some more conniving. If Angelique could get Josette to marry Nathan Forbes barely a week after being widowed, it would be quite a coup for Angelique, and I think she could even consider hitting the professional witch circuit.
It's a pity that Anthony George wanted out from the show. I have said elsewhere on this forum, and thought so more and more since I said it, that there could have been some interesting scenes if there had been no duel, and if Jeremiah had lived. Jeremiah and Vicky had been showing signs of interest in each other before Angelique starting doing her thing, and it is conceivable that, with Josette unavailable for marriage, Barnabas would have noticed Vicky's existence. Things could have gotten rather messy, and with Angelique stirring the pot, who knows what strange situations might have developed before the 1795 storyline was through?
Still, the duel gave us that amazing scene out on the terrace, with Josette telling Barnabas flat-out that he was a monster. The announcement of the marriage of Jeremiah and Josette would have been difficult for Barnabas, but at least by the time he heard it, he couldn't have been taken completely by surprise. And at that point, at least Josette didn't seem to hate him. Now suddenly he hears from her very lips that she can't stand him, and she never loved him. And he still loves her. Absolutely horrible for him.
By the way, it was a very irregular duel: no seconds. Sheriff Patterson would not approve.