Phantasma_ No, translators are replaceable. Although I would like to say no (after all I am a translator and I obviously wish to be irreplaceable), it is the truth. Good translators are hard to replace, but they are still replaceable if you find a translator as good as the original one, whether one can be found is another thing. There have been many books that have been picked up by translators midway. Quality might drop, quality might increase. It's a mixed bag, but it just means that the translator can be replaced.
Even in professionally translated books, like the the Three Body Problem trilogy, the middle book was translated by another translator, before the famous Ken Liu continued the third part. But the author cannot be replaced.
Why? Because the author is the one who even wrote it, without his ideas, his writing, there wouldn't be any work for the translator to work of. If the translator writes something, they become an author, not a translator.
Actually, Privilege now gives money to the translators without switching to a royalty agreement. So they do get additional pay beyond their fixed rates. So it's actually going to be a popular choice. Previously, translators were just translating at their contracted speeds since there's no incentive to go beyond the requirements, which is also why they can sometimes disappear.