Poor excuse for not adding things in DF.
Not adding things to DF lengthens the life of the content. In my experience, it's harder to put a group together than actually playing the content. If SE placed Final Coil in DF (even w/o nerfs), more people would be able to clear it and run out of things to do quicker. SE doesn't want this.
There are many good players that don't have time to commit to a set schedule and have no static group to clear content. DF will be helpful with this. It's an automated and more efficient PF if you're not bothered on who you'll be grouped together with. I've yet to hear a good explanation on why new content are not placed immediately in DF aside from "mechanics too hard, randoms will just fail". Trying content with random people on your own preferred time is better than not being able to try it at all.