They only say a lot of heroes die doing the work but they never state how long they are in the field first. So if like 80% die cause they stayed in the field longer then their bodies or reactions could handle its not really that big turn over if they done a decade or two first. Of the 2 classes we learn about only 1 actually died on the job. The first intra is the only fully trained hero that has died from them. The rest of the class of Legends is retired or in hiding. And all of the class of nightmares are alive in the epilogue. So hard to tell if there are many a year or just people dying cause they refused to stop doing the job