I don't see them dropping to the same levels as England, India, Pakistan, and South Africa but I do feel they will become less consistent than they have been.
They will still dominate though, they have been at the top for a long time going back past the Waugh brothers - the fact is they will produce someone to step in as if they haven't done already.
Every team has people retiring though just look at India who will loose their 3 big batsmen over the next few years and the likes of Kumble. Pakistan have lost Inzy and the likes of Yousuf and Younis aren't getting any younger plus Shoaib hasn't even been rewarded a new contract. England can have a decent first 11 but fall a bit short after that - too reliant on the likes of Flintoff, KP, and Vaughan's captaincy.
Can always hope.