There did have some people still waiting for GC even they have filed it many years ago. My school official told me there was one in my university who filed 5 years ago (EB1B).
There usually are some reasons for that, and 排期 won't solve their problems. My guess is their cases will be processed and sent back to the waiting list again. So few cases are approved. and so the date advanced even there are only a limited visa numbers (about 10k a year for employment based).
