The Dior Dilemma

[Raynor]

Tuor did not become an elf. He was "numbered among the elder race" - dwelt with them, was part of their population in Aman, shared their fate,

Then what does Tuor become? Does he still remain a mortal?

In letter #131, Tolkien states that the nature of Men "could not endure" immortality, and it is apparent that this quote is in refference to letter #156, where it is said:

The view is taken (as clearly reappears later in the case of the Hobbits that have the Ring for a while) that each 'Kind' has a natural span, integral to its biological and spiritual nature. This cannot really be increased qualitatively or quantitatively; so that prolongation in time is like stretching a wire out ever tauter, or 'spreading butter ever thinner' – it becomes an intolerable torment.

In my interpretation of these, no Man can be immortal.

IIRC, Tolkien comments in the letters that the view of Man that they were or can, or should be immortals is a lie induced by Melkor. Concerning this subject, Tolkien notes in his comments on the Atrabeth that (emphasis added):

He remains, nonetheless, in the opinion that the condition of Men before the disaster (or as we might say, of unfallen Man) cannot have been the same as that of the Elves. That is, their 'immortality' cannot have been the longevity within Arda of the Elves; otherwise they would have been simply Elves, and their separate introduction later into the Drama by Eru would have no function.

If I can rephrase that: if a Man is immortal, then he is simply an Elf.