Official Vanadiel Census
looking at this you can 40% of the player base upto or beyond "emptiness bleeds" with roughly another 30% of the playerbase scattered along at various points before that.
While those figures only represent people who have started those missions and not cleared them it does still show a significant chunk of the playerbase was doing cop. and if you look at "the last verse" almost 25% of players had cleared it entirely.
this census is also the 2009 one so the figures represented are before abyssea came and the leve lcaps were removed. so COP particiapation was actually quite high when you look at it.
note however i'm not saying my numbers are 100% accurate because SE simply isn't as open with information as it once was for one reason or another.