I might be wrong, but i wouldn't be surprized if Disney had at least a couple "back up voices" for their major characters (Mickey, Minnie, Donald, Goofy), so their probable "official" voice is more than likely already narrowed down considerably, for any of those characters.
Since they'd presumably be getting someone to take over mickey on a long term basis(read: as long as they can possibly do it), Disney has every reason in the world to be practical about this and not rush into a decision. They have enough stock audio of Wayne to fulfill any short-term needs right now.
What is disney gonna need new material from Mr. Mouse for anytime soon anyway?
Mickey's voice is used within the theme parks from time to time. Mickey Mouse Clubhouse has already been mentioned. Square-Enix has given King Mickey more visibility and has more Kingdom Hearts projects in the works.
New shows within the parks feature Mickey's voice.
And it happens a lot more often than you'd realize, because in addition to DLR and WDW, Mickey turns up speaking English at Tokyo and Paris fairly often, too - even occasionally at HKDL.
So yeah, Disney needs a new Mickey - but they've been pretty protective of these characters, and whoever they choose will almost certainly be expected to carry the role for the long haul - so the thing takes time.
And personally, as much as I want a new voice, I'm glad they weren't just sitting there ready to whip out a new name. It would have felt really disrespectful.
You don't need me or my ham, cheese, cheese, cheese, tomato, cheese, ham and tomato sandwiches. And cheese.