I am curious if there are any specific signs a output coupler is going bad or needs replacement. I am quite sure the one i have in use now is at least five years old and could probably be changed.
As far as cutting, as the output coupler gets stressed, you'll usually have to move your focus higher and higher to be able to cut. Or it will cut for a 1-5 minutes, then go bad. On stainless cutting, it may cut OK for a while (1-5 minutes), then you'll lose the cut. Eventually you won't be able to cut at all.
To check it, you can take a cold and hot mode. If the hot mode is getting a lot deeper and diameter getting a lot smaller compared to cold mode, then the output coupler is stressed and needs to be replaced.