system
Given the following post, respond as if you're commenting on Reddit:
human
[LotR] Why did Illuvatar make it so that magic would gradually disappear?

Given that the elves, one of his two favorite children, suffer greatly by the gradual diminishing of magic, why wouldn't he make magic constant and undimishing?
gpt
It's kind of the other way around; magic is diminishing because the Elves are diminishing. 

Morgoth messing with the creation of Arda led to a concept called Arda Marred, which is basically the corruption that Morgoth forced upon the world. The Valar abandoned Middle-earth and colonised Valinor instead, allowing it to be the only place on Arda to be free of Morgoth's corruption. That's why the Valinor brought the Elves to Valinor, because only there could their bodies and spirits not suffer the effects of Morgoth's taint upon the world. This is because their spirits are tied to Arda in a way that Men's spirits are not, so that damage to Arda essentially also means damage to the Elves. 

When the Noldor returned, they were essentially at full power. Their spirits were unmarred and they'd learned plenty of things from the Valar, plus they'd all seen the light of the Two Trees of Valinor (which Elves born during and after the First Age would never see). However, as time passed, the Elves in Middle-earth slowly diminished, even after Morgoth's defeat. Due to Arda Marred, Elves in Middle-earth would slowly "weaken" over time, eventually resulting in them travelling west to Valinor. This can take thousands of years, in the cases of people like Cirdan or Celeborn, but it was ultimately the inevitable fate of all Elves. 

That's why the Three Rings were created - so that Elven rulers would delay the passage of time and decay in their lands. This is why places like Rivendell and Lothlorien are so magical, because Elrond and Galadriel are using their rings to artificially enhance the beauty and majesty of their lands. 

Once the Three Rings lost their power, most of the remaining Elves left Middle-earth shortly afterwards as they could no longer stay. Even if they wanted to stay, their bodies would eventually weaken to the point of which they would become harmless wraiths. 

So to put it bluntly, the Elves brought "magic" to Middle-earth from Valinor. As the Elves weakened over time, as did the "magic" in Middle-earth.