Why did Tolkien choose to end the main Royal line of Durin's folk?

Spoilers for the Hobbit ahead

At the end of the Battle of the Five Armies, the prominent casualties claimed by the War on the heroes' side are Thorin Oakenshield and his two nephews/heirs Fili and Kili. The three deaths mean that the line of Thror and Thrain is extinguished and the Kingship of Erebor passes to Dain II Ironfoot.

Does Tolkien discuss why he did so in any letters/writings? Is there any literary or historical (as important for the events of LotR/eventual defeat of Sauron) meaning or importance to the wiping out of this particular line, in Tolkien's lore?