It's interesting that Brian actually said he felt guilty for not recasting Kermit prior to the Disney sale. To those who keep asking why didn't he do something about it a long time ago, I think you're now seeing why. It doesn't look good. People are mad at Disney about this, even if it might have been completely justified. If Brian had done it, not only would people have been mad, they would have seen him as incompetent. He says this was already a problem in the '90s. Imagine the press if less than 10 years (maybe close to just 5) after Brian personally chose someone to be the new Kermit, Jim's signature character, he had to fire the same guy. JHC was struggling enough with the Muppets already. With people questioning his business/creative decisions he didn't need need questions about personnel decisions, especially considering what a big deal was made about choosing Steve. It would have called his personal judgement into question.
I'm impressed that Disney worked so closely with Brian and the Hensons to re-cast. There was absolutely no reason they had to. I'm sure part of it was to soften the blow to their public image, but it sounds like they really involved Brian in the process more than just a brief consultation. It could be posturing, but it appears that Disney really wants to respect the legacy of Him Henson through Kermit.