Is Thorin Oakenshield's death necessary

Answer 1

This is a question asking for an opinion. As a result, you may find dissenting views.

Answer 1: Yes - Because of the War of the Ring

Long and metaphorical answer: Yes, because everything happens for a reason, even if we do not see why at the time. For example, if Thorin had not died but become King Under the Mountain, then Dain would not have become king. Many years later in the War of the Ring, the dwarves would have fought with different strategy. This would mean that the end results would be different, possibly meaning that the orc armies sent to fight in the North would have been in Mordor and would have caught Frodo. Then, the ring would never have been destroyed and the fate of all Middle Earth would be completely changed.

So bottom line, yes, his death is necessary.

Answer 2: Yes - External Mirror to Internal Corruption

A very important theme in The Hobbit is the corrupting power of incredible wealth. The wealth of Erebor corrupts Thorin as it had corrupted Kings Thror and Thrain before him, rotting his soul. His mortal wounds in the Battle of Five Armies make his exterior mirror his interior and brings about the reconciliation between him and Bilbo. If Thorin had lived, it is likely that the clarity he achieved would not have come. Additionally, the ascension of Dain II Ironfoot allowed for a new relationship to develop between the Elves of Mirkwood and the Men of the rebuilt City of Dale on a very different page than how Thorin had left those relationships.

As for Answer 1, even if Thorin had led a different strategy to defend Erebor in the War of the Ring, it is inconceivable that Thorin would have accepted the Dark Lord's offer of Union. Thus, it would be requisite for Sauron to send an army to the North, even if that battle had gone differently. As a result, the reasons discussing the War of the Ring are unsupportable.

Answer 3: No

As the author Tolkien had the power to write Thorin in such a way as for him not to die. Not much else happens in the narrative after his passing and there are no things that Dain does that Thorin could not have done instead. Is it more poetic that Thorin dies defending "his gold" than in pursuing it, certainly, but it is not necessary.