Why did Arwen do that??

[Rimbaud]

Tolkien's point evolves from the fact, IMHO, that despite these vast emotional motivations for remaining in Middle-Earth, several major characters instead make their departure. In some cases it could be argued that The Gift of Men, tall dark chap with a scythe, was hugely appealing to grief-struck immortals, not normally having the option; in other cases it can be seen that the allure of the West, of being next to heaven, was greater than that of any earthly ties.

Few here, I would imagine, are suggesting that Tolkien neglected to consider Sam/Arwen's attachment to their offspring. Indeed, I very much suspect that the appreciation of that love, and his concept of limits on love imposed by mortality, formed the basis of his message here.

Perhaps Tolkien's point here is a more religious one than a glance might convey. That despite the incomprehensible love between those separated through the course of the stories, all find their final solace elsewhere.