Well, I said phase out, not remove completely...
You'd think those with limited free time would be more keen to make friends and essentially get instant access to content... I mean unless you have a rigid static, you're waiting no matter what... Even with Tank and Healer, I rarely get instant queues...
Regardless of how much time I have, I would rather wait to form a group I know, be that personally or that I know is going to adhere to whatever standards were detailed in the Party Finder, than wait for a complete wild card in the Duty Finder. At the very least I can Blacklist any problematic player picked up in Party Finder, no such luck in the Duty Finder.
All they need to do is slap a bonus for doing the dungeons without the Duty Finder. That makes it worth the non-existent extra wait. That's honestly a much better concept for "Free Company Content" than whatever Diadem (Hard) was meant to be. Certainly some details and balance issues to be ironed out, but that is something that leans much more to creating a social environment where groups can play their way, than the Duty Finder, which has created at best a convenient anti-social lobby, and at worst a toxic one...
Something like dungeons with multiple paths, puzzles, and optional bosses, shouldn't be "impossible". They only are because this game practically forces use of the Duty Finder, so content has to be standardized around the lowest common denominator. Dungeons will be done by speed runners and people interested in stopping and reading every note in Tam-Tara (Hard), and the Duty Finder pairs both those players up.