“The industry took no major issue with the 2017 proposal, though some airlines requested longer compliance times. They said the rule would affect more than 220 engines because airlines often swap blades between engines.“
Fan blades are often removed to repair minor damage (bird strike, FOD, etc.) - this is likely more common on 737/CFM56 due to the low inlet on the 737 and the relatively high number of cycles the engines see relative to wide body aircraft. How this is done is operator specific - some change blades as an engine set, some replace the damaged blade(s) along with the blade 180 deg. opposite with a matched blade to retain fan balance, still others will only replace the damaged blade(s) then do a fan balance run.
Fan balance is very important for the CFM56 on the 737 - the engine is so close coupled with the wing that any fan imbalance will cause annoyingly high vibrations in the passenger cabin.
But the main point is that if an operator has 10 engines that would be affected by the AD, the fan blades originally from those engines might be spread around many more than 10 engines.
<edited to address Concours 77>
While there will always be a small amount of wear between the blade dovetail and the hub, special coatings and lubes are used to limit the wear, and there are strict limits on the allowable amount of wear (it's not much - a few thousands).
However, having been involved in several FAA AD's over the years, 8 months and it's still not released is inexcusable. AD's by definition affect flight safety. Normal flow time for a 'routine' AD is a couple months - seldom going much beyond 3 months even if there are lots of comments (emergency AD's don't go through public comment and can go out in a few days). While getting the blade inspection AD released may not have affected this accident, it's pretty clear to me that someone in the FAA dropped the ball.