Letting someone else direct is not going to necessarily speed up the process, it would actually delay it further since they would have to put it out to tender and then go the process of hiring one. And if there isn't a completed script any 'new' director won't be doing anything except sitting and waiting for it to land in his In-tray.
While I don't quite understand the reason for the delay in his confirmation of returning to the director's chair (it's seemed inevitable since the first film came out and thus pointless to keep it rumour to this point) the main issue is whatever the 'real' current state of the project is.
Which is not being precisely revealed.
I'm starting to suspect that one of two things is on the cards. The film 'is' actually at a more advanced state than can be determined just now and the date can be made............or the film is going to have to move because they will have to rush it at this point to get it ready for the scheduled date.
Rushing to make a pre-fixed date is rarely a wise move.............but one that studios will do (TMP did it after all) when it suits them.
Personally, I can wait til December 2012 if need be. But 2012 is already a very crowded year - both during the Summer season and the Christmas Holiday season as well. There 'could' even be a viable argument for waiting until 2013 when there's much more room to move just now.
But that's for Paramount to decide in conjunction with the director.
'If the Apocalypse starts, beep me!' - Buffy Summers
'The sky's the limit.....' Jean-Luc Picard, 'All Good Things'
courtesy of Saquist