If Taint were just a physical malady, I would be inclined to agree with you. But it isn't. It's a spiritual and moral affliction that spreads like a physical disease, and can end up with a soul ripped from the cosmic cycle, stripped of any chance to fulfill its destiny, and condemned to Jigoku's embrace.
This is actually pretty overblown, and mostly present to give an obvious source of cheap drama and lift the burden of writing complex evil characters. Otherwise, the Tainted character can just have some sort of "divine intervention" and get rid of her Taint by killing a random oni or he can just climb out of Jigoku and return as a 100% pure spirit-samurai to fulfill his destiny.
Also, I would like to point out that while Locust Shell has a fair point, being reminded to one's "mortality" can be quite counter-productive in a fantasy samurai game. You are supposed to piss all over your mortality in this genre, after all.
Do you mean that you think my comment is overblown, or the way Taint is handled as largely incurrable is overblown on the part of the writers?
The latter. It is overblown by the writers (thus giving the kinda-sorta-false perception you described) to cover up how a lazy excuse it is for cheap drama and one-dimensional evil villains. But then, it is just as bad and incurable as the story demands it, so there is no point saying that being Tainted is equal to being irreversibly doomed - it just means that the Tainted character is in the need of some random *sspull to get rid of his/her curse. Unless the character likes his/her Taint, but that's a different can of worms .