On the inability to get to Valinor and its different forms over the different ages:
1. At the First Age, after the Vanyar and the Noldor and eventually Olwe's Teleri had gone with the Valar to Aman, the shipbuilders in Beleriand, that are Cirdan's people, simply weren't technologically advanced enough (by ancient standards) to build ships that were robust enough to sail all the long way to Valinor, and so Aman was inaccessible to them. When the Noldor came and the sun rose for the first time, they began gradually to improve their ships, but for some time all the ships that tried to sail to Valinor still failed because they couldn't stand the terrible enchanted isles that were in the way. The Edain got to Beleriand from the east in a quest to the "Land of the Gods" in the west, but discovered that there simply weren't any ships that could bring them (nor the elves at the time) to that land. Until one day, Earendil's ship made it successfully!
2. At the Second Age, at the days of Numenor, Men were forbidden by the Valar from making a voyage to Valinor. But unlike the days of the First Age, the possibility of doing so existed, and many elves in Middle-Earth indeed did it (because of Sauron). Until one day, Ar-Pharazon's ships made it even though it was forbidden!
3. At the third Age it is again impossible for Men to sail to Valinor, just like at the First Age. But now for a different reason: Valinor is in another dimension. A dimension that is unseen to mortal beings and hidden from them. A "magical" dimension.
Let's analyze Tolkien's writings as an imaginary pagan mythology, rather than as fantasy books or an imaginary history, as if we analyze a fictitious mythology of a real human tribe:
We see here three stages at the hiding of the "Land of the Gods" from humans:
1. It exists in the world, but in a place humans cannot physically go to, simply because they don't have the means to.
2. It exists in the world and accessible to humans, but the gods themselves declared they don't want any humans in their kingdom, and humans don't dare to go there lest the gods would severely punish them. In other words: the sailing to the Divine Land is a religious taboo.
3. It exists only outside of the world.
I think Tolkien based it on a possible development of a typical real-world pagan religion:
1. At first, the pagan people that believe in that religion are some primitive tribes from four thousands (or three thousands or five thousands) years ago that don't know much about the big world that outside of their own land and its near lands. They think, based on some ancient tradition, that the world is flat and that the gods dwell in the Western end of it. At this stage they still don't have ships that can sail very long distances, and they don't think they ever will: technological advancement simply isn't something one thinks about at their primitive culture. They also aren't curious. So it never even comes to their minds to try to find the Land of the Gods.
2. After some hundreds or thousands of years, the technology of that civilization becomes more and more advanced, shipbuilding included, and the people of those once-primitive-tribes, that are now important countries, begin to think seriously and philosophically about their religion and develop a smart theology. They also get to know religions of other peoples from far-off lands. When they have relatively strong ships, the theologians of the religion of that nation realize that there is some chance that maybe the Divine Land doesn't really exist, that it is only symbolical/allegorical and the gods are just personifications, but they know the whole foundation of their dear ancient religion would die out if it would ever be found out by the dumb people that need physical gods to believe in, so they declare to the less wise believers that there is a reason the gods had never invited Men to their immortal land and that they forbid them even to search for it. They hide the secret even from their students, the theologians of the next generation!
3. Some of those countries are now big empires (or they are one united empire) and they go to many voyages to conquer other countries, many of whom are distant, and they notice they never go near any magical land when they sail in the western direction. They discover their cosmology all those years was wrong and that the world is round, so it has no a western end. They start to notice that the gods hadn't spoken to them for thousands of years and doubt they'd really punish them if they ever come to their land. They also start to think that maybe... just maybe... other religions are true, not their birth one.
So the theologians solve it with a major change in the cosmology:
They simply remove the gods and their land to an unseen, magical sphere: The spiritual dimension is introduced to the religion, and the gods change from simply very strong immortal Men that can do "hocus pocus" to personifications of abstract powers. The pagan religion is now a shadow of what it once has been: more spiritual, yet much less sensed and understandable by the simple peasant.
This is what the removing of Aman outside of Arda symbolizes to me: the "spiritualization" of the religion and its once-physical gods. The religion becomes a shadow of the very sensual and "real" religion of the primitive tribes from thousands years ago.
4. The fourth stage is the ultimate death of the religion: people simply stop believing in it. To me this is symbolized by the departing of the high elves from Middle-Earth. Because those were the high elves who introduced the Valar to the Edain who came to Beleriand seven thousands of years before.
I may add a "zero" stage: the ancient tradition of the primitive tribes from stage 1 about the gods and their land are actually based on a yet more ancient time when their ancestors had the ability to recognize the divine in the physical nature, because of a greater level of imagination. An ability that the distant memories of its days are represented by the Eldar. An ability that its losing is symbolized by the destruction of the Two Trees of Valinor.
