It’s been a long time since I read those books. If my googling & memory is correct, they at some point during the 74th Hunger Games announced that two victors from the same district would be allowed, then revoked that to ensure Katniss and Peeta would kill each other, then backflipped to allow the original agreement when they threatened a double suicide. Have I got that right?

I haven’t put a huge amount of thought into this, but I suspect their iconicness would be much lessened, and thus their ability to inspire rebellion in the future.

If they never did the whole 2 can win rule, you still get a powerful figurehead in Katniss, or Peeta if you think he’d actually kill her. The lack of blatant unfair behavior would hurt the resistance in recruitment though. The resistance might still succeed, but it would take even longer. The trauma of having to kill the other could also leave the survivor broken which would be bad for the resistance.

If they announced thev2 can live, but committed to taking it back, the double suicide would be a useful tool for the resistance. They wouldn’t have Katniss as a weapon, but her martyrdom would be almost as powerful. The key here is that a successful resistance would just be a different dictator.

However the author chose? This is fiction, there’s no logic to it.