Taking filming the movie out of the equation... well, look at it this way. Eric does most of Frank's old roles except Cookie Monster, who's played by Dave. Steve Whitmire has a great deal of Jim's roles, but Bill does the other half, and Eric was Guy Smiley on occasion. Just because one performer takes on a bunch of another performer's roles, that doesn't mean that multiple puppeteers can't be recast as one performer's cast of characters.
I feel mixed about whether one performer should take over everybody from the same performer. Back in the 1990s and early 2000s when recasts were occurring they didn't stick to just one performer for each character, unless maybe there were multiple characters who had the exact voice. And even then, until 2005's "The Muppets' Wizard of Oz" the same-sounding Rowlf and Dr. Teeth were recast with different performers. Heck, at first Eric Jacobson wasn't even performing all of Frank's characters. In addition to not taking over as Cookie Monster, at first he and John Kennedy were alternating with Animal (since Kennedy was also performing Dr. Teeth at the time it would have been awkward if he had two Electric Mayhem members... and then he performed Floyd a couple times after Baretta became Dr. Teeth's performer and Jacobson was performing Animal all the time) and Kevin Clash was performing Sam the Eagle, though it seems like even in the early 2000s Scooter and Janice were kept with the same performer (except for in The Muppets' Wizard of Oz, though I suspect Brian Henson originally intended on dubbing their voices later, before hearing how good the performers were -- though in Janice's case I wouldn't be surprised if it was because her speaking scene was cut from the original broadcast).
And then in 2008 they had Matt Vogel taking over nearly every Jerry Nelson character, with a few exceptions, while David Rudman took over as Scooter and Janice, though I'm surprised that following John Henson's departure from the role that Vogel took over as Sweetums instead of Rudman.
Of course, it's amazing that one person can take over all (or nearly all of) the characters from a performer. It's amazing how Eric Jacobson can do most of Frank Oz's characters well (and he can do all the Frank Oz characters he does very well), though I feel it took quite awhile for him to do Miss Piggy well. Ditto for Matt Vogel with most of Jerry Nelson's characters (I feel Uncle Deadly sounds more like his original self when he's shouting, but when he's talking calmly it sounds a bit more different). I would have thought that just because a performer can do one character (especially the voice) well doesn't mean they can make everybody from the same performer sound the way their performer did (I know, performance comes before voice). On a side note, when I first learned that Steve Whitmire took over as both Kermit and Beaker, I thought he had also taken over as EVERY character originated by Jim and Richard as well (I know, it would have been hard for him to be both Statler and Waldorf).
Another thing that makes recasting all characters from one performer to multiple performers a good idea is that it can open up to seeing certain characters interract more. If Matt Vogel takes over as The Count and Tyler Bunch continues as Herry (which according to Twitter he assumes won't happen) that could lead to some interaction between The Count and Herry, for example. Though it seems like there still hasn't been much interraction between Jim's characters even with multiple recasts. I think the only such interraction we've gotten a bit more of has been between Kermit and Rowlf (ironically, it seems Kermit and Rizzo have interracted more since Steve has been performing them both, and I think the same can be said about Kermit and Beaker). Of course if a performer gave multiple characters the same voice (like how Jim gave Dr. Teeth, Rowlf, and many others the same gruff voice), they probably should be recast to the same performer, since that performer can do the voice well. Even if John Kennedy was still performing Dr. Teeth today while Bill Barretta continues as Rowlf, it might seem weird to see those two interract.
Tyler's twitter says this was a one-time thing and he most likely won't be doing him on the show.
That's a shame, considering he did do the character well. But who knows, maybe his performance impressed Sesame Workshop enough for them to decide to let him continue as Herry (but only time will tell).