In the Whitmire part of the interview he makes it seem like only Whitmire can play Kermit like you can never recast the role. that to me is arrogant and I totally disagree the real Kermit is JIM HENSON!
Another interesting thing is that Disney offered him 'consolation prizes' like a Disney Legend award if he signed an NDA. (On a side note, this year's Disney Legends are a joke when you consider the 'Disney' part of it.)
Now, to your point...I think legacy Muppets are very hard to shape since the Muppets are part of the Muppeteer's own personality. Grover, Piggy and Fozzy are not just characters, they're part of Frank Oz.
Now when a performer 'inherits' a character, they can't 100% duplicate the character because they're obviously not the same person as the original performer. The character will then mostly be like it was with the original performer mixed with the style of the new performer. This way we have characters like the Jim-Kermit, Steve-Kermit, Frank-Grover & Eric-Grover. Honoring the characters and original performer as much as possible but adding a bit of the new performer to fill in "the gaps".
That being said, the roles can of course be recast but recasting does change the characters a bit so it's something that needs to be done as few as possible with the 'main' Muppets.
Steve isn't Kermit, but he's a big part of the current Kermit and a part of the current Kermit is Steve.