here are 2 good reasons to forget the idea.
1. Its irreversable... dont try to talk about a godly cure, there is no precident, and honestly no reason to start one. They are despised, even by necromancers.
2. You can barter or trade or go into town as a lich. It breaks the game right there, there is no point.
ill think of more later, maybe.
It doesn't need cure. As long as it's clearly stated to be a permanent change, and you have decent abilities to enjoy the game (Speed attribute is not about 5 and so on). It's a just fine option.
Also: People are still trying to uncover more info and secrets about Liches and Lichdom, so it's not completely impossible to have a cure. It just can't be known by every priest and librarian. It can be some ancient knowledge only Liches can get access to.
Liches have multiple stages. At first the bonuses and drawbacks can be small, but as you dwell deeper into the mysteries of Lichdom, the pros and cons become more extreme. You can have humanlike attributes at first, cover the first skin deformations with clothes and talk like a sore throated person. Later you need illusion and distance to be able to play civil. In the end everyone will flee or try to kill you, except maybe a few Lich fans/too intimidated to fight or flee around the world. Think like cleaning a city to have just 3 people who can stand a full Lich, you can kill them easily if you want. That is the horror and destruction I'd imagine it to be. You don't have to trade or barter, just take, Liches don't even need nor can't carry most of the junk townspeople have.
It shouldn't be as easy to move bethween stages as it is with vampires. Now we know it's easier to evolve as a Lich than to reverse towards humanity. Some complex rituals could allow you to become a more powerful Lich, and even more complex rituals to reverse the effects. But the info how to do them is not brought to you with popups as soon as you first hear about Lichdom.
And I don't care about Lichdom so much.