Question: Was Arthur always supposed to die? Or was he originally supposed to survive?
Answer: Arthur dies in both endings; it's a question of how.
If his honour is high, he will die from tuberculosis. If his honour is low, he will be killed by Micah.
Other mistake: One of the things that drives the game's main story is that the gang needs to acquire enough money through various heists in order to escape from the U.S. government and live in Tahiti. However, when exploring and doing side missions, Arthur can acquire tens of thousands of dollars by himself, which could easily pay for everyone to flee without having to rob banks or trains.
Answer: I don't know a definitive answer, but given that the story of this game is ultimately about the fall of Dutch's gang, and that the protagonist of the original Red Dead Redemption game also dies near the end, I would bet that the developers always intended for Arthur to die.
Phaneron ★