The Elves' Mithril Retcon in Rings of Power is Just Plain Ridiculous
The Rings of Power sure featured a lot of dumb retcons and poor decisions. But what they did with the mithril is a strong contender for being the dumbest of them all. At the least, it can be ranked in the top three.
Both in the original Tolkien's legendarium, and in Peter Jackson's movies, mithril was just a metal. Certainly, its rarity and its highly valuable properties – it was said to be as easy to shape as silver, but much tougher than the best steel, when properly tempered – gave it semi-mythical status and turned it into an object of rampant greed, which indirectly led the Dwarven kingdom of Khazad-dûm to its destruction, when its denizens stumbled upon the monstrous Balrog in their attempts to mine more mythril. But by itself it had no magic properties.
However, in The Rings of Power mithril happens to be a super-mystical metal, which was created by a legendary battle and which somehow happened to be the vital component for continued survival of the Elven race in the Middle-earth.
Now, from the book viewpoint this is nonsense.
While Tolkien did not say straight-up (outside of his notes and letters) what laws of the world exactly forced his Elves to leave Middle-earth, the text of The Lord of the Ring implied it quite heavily that outside of Valinor, or Middle-earth lands where something like the power of the three Elven rings was used to replicate the same "timeless" condition, Elves simply grew weary of eternal life.
Not unlike what happened to Bilbo after he wore the One Ring for many decades, except the process was much slower for beings naturally adapted to immortality.
There was no limited amount of magic juice that could suddenly threaten to run out within a space of a year or two and needed mithril to be replenished.
But what's more important than faithfulness to the source material, the whole mithril retcon was just a plot contrivance. The showrunners wanted the Dwarves in their show, and they invented the whole storyline of Elrond going to the Dwarves for mithril, and they could not think of any better justification for Elrond to do so, and for Prince Durin to defy his royal father for the sake of Elrond, than turning mithril into a magical McGuffin of overwhelming importance.
Even though they needed only slight tweaks to do the same storyline while keeping mithril as "merely" a jealously hoarded object of greed.