There is a saying that power corrupts, and absolute power corrupts absolutely. One Ring from Lord of the Rings is an embodiment of this metaphor. It is that one part of this story where this book is not just a plain fantasy like many other books but rather a literary fiction that describes real world in a metaphorical allegoric way.
One ring is a metaphor of power that one might wield in life. CGPGrey would put it as: take the throne and it will act upon you. One ring is not just a piece of jewelry with some magical powers, created by Sauron. Ring is an embodiment of his power over Mordor and orcs. All that makes one a Dark Lord is contained in the ring.
It is the ring that gives one power to rule Mordor. Essentially the ring makes Sauron. Even if the original wielder and creator, Sauron is defeated and killed, the ring will still exist. It will be able find someone else to be next Sauron and rule over Mordor. Thus, Mordor and Sauron's system will endure even if he himself is dead. Sauron managed to create for himself an immortality of sorts.
Notice how Sauron just disappears the moment Isildur cuts the ring from his finger. Sauron is not a person with evil intentions. He is an embodiment of the system. For as long as power exists and used, the Sauron will exist.
The better one understands what this ring is and what power it has, the faster this ring will turn them into the next Dark Lord. That is why people like Gandalf and Galadriel refused to take this ring themselves. They knew that if they take the ring, it will corrupt them and by the time they will reach Mordor they will be there not to destroy the ring, but to rule as Sauron the second.
Thus, One Ring produced a catch 22 problem that prevented forces of good from defeating evil. As long as One Ring exist so is Mordor and its system. However, if one of the knowledgeable and powerful were to take this ring to destroy it, the ring will corrupt them and turn them into next Dark Lord. No matter what they do, the evil cannot be defeated. Elrond hoped that humans, clueless of what One Ring does might be willing to destroy it, but humans were too greedy to simply cast the ring into the fire.
The solution came unexpectedly, the Hobbits. Hobbits were too simple and unable to understand the true power the One Ring had. Even when a greedy homicidal hobbit, Smeagol (Gollum), had it, he could not utilize most its power and did not manage to become next Dark Lord. Bilbo got this ring by accident and its corrupting powers did not manifest in him at all. Gandalf noticed that and came up with the plan to destroy the ring.
Since by that time Ring corrupted Bilbo enough to at least not wish for it to be destroyed, Gandalf made him pass it to Frodo who was so oblivious as to what this Ring does, he would have no objections to destroying it. Then there was a need to arrange a for an escort party to make sure he makes it to Mount Doom alive.
In real life power works much in the same way. Take the throne and throne will act upon you. Reality of having power makes people into evil lords.
That is why for example attempts to root out corruption in post-Soviet states often turn anti-corruption activists and revolutionary leaders into corrupt dictators the moment they take power. System is so cunningly crafted by some original dark lord in the past, it manages to recreate itself no matter who takes power.
Tolkien did not make this up, he just described how it works.
If you think this is farfetched then ask yourself that: if you had the power to rule over something and use this power to your advantage, will you just throw this power away out of moral considerations and desire to be a good guy, or will you use it for personal gain or perhaps to achieve some moral vision of the world you have dear...
In the end only well-designed systems can endure good governance and decent life. Unchecked power and opportunities for corruption will lead to dark lords.