Also, there's more to a character than just a voice. It's about the "performance". And the current Muppet performers have been giving great performances.
Who cares if the current voices sound slightly different from the originals? Don't tell me people prefer stale copies.
In the case of Bugs Bunny and the Beatles album... it's not like the guy who voiced him didn't try. While his voice didn't match exactly, he was able to get the speech patterns down right. However, unlike Disney or the Muppets, Warner Bros doesn't have quite the same consistency when it comes to recasting a voice. Mainly due to the fact they don't want another Mel Blanc situation and keep them from anyone asking for outrageous pay raises. While no one can ever replace Mel, there are voice actors that do characters better than others. What is strange, though... the voice actor never played Bugs outside of the Beatles album, at least to my knowledge. Plus, I find Jeff Bergman, Billy West, and Joe Alasky did a better job with Bugs anyway...
As for everyone else, as I said, Wayne WAS Mickey for me. he did the best job out of the bunch, and really managed to get the character to emote more than even Disney did. Donald and Goofy I always loved, but I feel that Mickey's best projects were, ironically, later ones. Though that's not to say stuff like Mickey and the Beanstalk wasn't great. Plus, the thing I like about Donald's recast was, just like the original, he was also an animator.
But with all recasts, sometimes it takes time to find someone spot on. The combination of Muppet recast is, who embodies the character best, who's most available, and who sounds the closest in that order. Sometimes they had to go through multiple puppeteers to find the perfect recast. I still wonder why it took that long to get David Rudman to do Richard Hunt characters, when he's been doing Richard Hunt recasts in Sesame Street for a while. I swear his Baby Bear is a vocal tribute to Richard (there's something about the character that screams Richard would have played him) as much as Bobo the Bear was Bill's vocal tribute to Jim. And that's, strangely enough, why he does a similar voice when he's Rowlf.