Given the current rate at which throat queens retire, JAV studios have likely done the math and concluded that the turnover rate is acceptable from a business perspective.
This is obvious, isn't it? We can talk about about throat queens, but at the end of the day this is a genre based on squeezing poor girls that aren't able to star softer films.
JAV is a ruthless business, and in this genre, the more the girl suffers, the more money the company gets.
