According to the rumors, Disney stopped doing Mickey's voice when his mother complained about how irritating Mickey sounded. Now, if that's a true story or just something animation students just say is debatable.
That's not true, of course. The only reason why Walt stopped doing Mickey's voice regularly was because Walt became way too busy with his ever-increasing responsibilities around his studio (working on movies, TV shows, Disneyland, etc.) to do voice-over work on a full-time basis.
But frankly, and I always shock the heck out of people when I say this... Wayne Allwine was the BEST Mickey there ever was! He was able to emote. I LOVED the cartoons on Mouseworks and House of Mouse and the Three Musketeers movie. I've always been a Goofy/Donald man myself, but Wayne did something with Mickey. I don't know what (besides emoting), but just... he was amazing. Never heard who he was recast with, though.
I agree that Wayne was the best voice actor Mickey ever had. He brought a lot to Mickey, bringing him into the modern world while maintaining the character's integrity at the same time.
From the bits and pieces that I've heard of Bret Iwan's Mickey voice so far, he sounds pretty good and so far has done the character justice. Even I have had a bit of trouble telling the difference between Wayne's Mickey and Bret's Mickey, even though Bret's Mickey voice is higher pitched than Wayne's.
But I'm guessing half of them who complain still think Brian's Kermit or something...
That seems to be one of the reasons why as well.
The casual fans also don't realize that there's more to a character than just a voice--it's about "performance". Even though Steve's Kermit voice sounds slightly different from Jim's, it's not bad. And most of all, Steve is a great performer as well.
If I had to choose between someone who can perfectly imitate a character's voice but has lousy acting skills and someone who sounds slightly different from the original but knows how to act, I'd choose the latter.
I again apologize for offending anyone. I just don't like it when people bash the current Muppeteers without thinking and they should at least be happy that the characters are still going strong today, even without the original talents.