It's really difficult to know what to say about all this ...
Disney have every right to recast if they want to, i get that. Matt is a superb choice, I am a HUGE fan of his work and I think he showed with Constantine for MMW and then Uncle Deadly for the TV show that he has been completely kicking butt and has so much energy and love for what he does.
It was amazing for the two fansites getting the scoop and they do amazing work and have a lot of love for the Muppets. On the other hand I feel it would have been more respectful given this is the first change of lead performer for 27 years if Disney would have put out a proper announcement to the mainstream press or at least given Steve the opportunity to announce first without getting into too much blame pointing, that he would no longer be working with the Muppets. Sometimes in these situations the talent concerned is given the opportunity to resign so in the eyes of the public they weren't fired, but I can understand why Steve wouldn't have wanted to be seen as quitting the Muppets and it doesn't sound like they have even been communicating.
The timing of releasing the information is very suspect and I can imagine has been carefully calculated. He hasn't been performing Kermit for a while and then a few days before a new video is released they get the info out. Either that is because they want to get the bulk of bad reaction out of the way first, or they want all eyes on the new Kermit video. They would have definitely known the news would go viral.
I don't know what the reasons are. Although Steve makes a point in his blog of talking about knowing the brand and characters and how special they are which points you there and that he maybe didn't approve of things they might want to do. I did fear the other reason was maybe money as although they are (from what i gathered) freelance performers I would guess they are retained and also that they were on good deals with Henson when they were used and i'd imagine Disney would have had to keep up those rates with Kermit's performer being on top scale. Given the projects with Disney haven't done quite as well as hoped I did wonder if the Muppets are maybe being given less budget and they might want to use Kermit more without being able to afford Steve. Could there be any concerns over his actual performances or style of performance in Disney's opinion? Who knows, but there is nobody who knows these characters better in terms of what Jim Henson's Muppets would say or do, other than Jim, Frank and Dave. But then again, do Disney want Jim's Muppets or to change them.
Of course there has already been some change, especially in the TV show where they were a little more updated for today's audiences, but then retro/nostalgia TV revivals have become even bigger now and it seems the public prefer them as they were so maybe a classic return to TMS may have done better. I guess it's the big question for Disney, where do they go with them, two only relatively successful movies, TV show that lasted just a season - I can imagine it's going to be hard for them to get the finances approved for many more big projects unless they pull in bigger ratings or money. Merchandising seems near non-existent, the profile isn't high in the parks, at least they are doing more digital content and with any luck that will do well given so much of how TV content is watched is changing at a supersonic speed.
Excited to see the new videos, and anything new they can bring us, with a slight fear if there is some kind of disagreement between the older Muppeteers and Disney over how the characters are being used doesn't lead to people like Dave and Bill following Steve. It's hard to swallow, especially as with Dave and Steve these were Jim's choices for the characters, but as a wise frog said to me elsewhere, creative differences like this happen all the time in media, and this is true, we are lucky up until this point there hadn't been much more turmoil in the Henson-Disney transition. The mouse (it seems) definitely made an effort to respect such a classic and beloved brand.