What's being lost is that what we may now cringe at today is some of those earlier performances from the 60s/70s were high quality at that time.
Jim, Frank and their team (including Steve) took uncharted territory in the field of television puppetry and raised the bar. They were the innovators.
People talk about how Jim was a genius because he saw the television screen as the puppet stage as opposed to a pre-existing stage on a tv screen. What makes this genius was not just the mere removal of a stage and using the bottom of the television picture as the thing the puppeteers hide under but the realization and constant tailoring of all the things that come with it - making puppets that could work in such a new medium - expressive flexible faces, seams that weren't so obvious in extreme closeup - the idea of a puppeteer working with a television monitor and being able to adjust their performance in real time was something no one had done before (or had to do). By the same token, the original Muppeteers had to learn how to deal with the new problems their solutions caused - yes, you could see via a monitor how high or low your puppet was on a screen, but you also had to go through years of getting used to fighting one's natural reflexes and learning that everything on the screen in front of you was reversed to your point of view (moving/looking left vs right)
They were learning as they went because they were crafting the original maps. They had to bump into the tree in order to warn followers "oh by the way, there's a tree here - duck down".
The performances then were outstanding because they were the best at what they did at that time. Not only did these already talented people continue to improve and up their game, but newer younger up and coming puppeteers also learned from their mistakes and acheivements. New Muppeteers have marvelous training available to them - they learn stuff in months, WEEKS even, what it may have initially taken Jim Frank and Jerry years to nail down. Plus with various education opportunities (school courses, puppetry troupe workshop) and easy access to video equiptment like camcorders and the like, one can train themselves how to work with a monitor a long time before they ever think of doing professional work.
The Muppeteers were at the top of their craft in the 70s. They're at the top of their craft now. But top of the craft in the 21st century blows away top-of-the-craft 70s era. One should expect better today because the bar has been raised.
Does that mean absolute perfection is expected of newer Muppeteers? No, there's a learning curve that comes with experience. That's why the pros get the big parts and the newer members do right hands, background characters, and work up to small parts and as their experience/talent grows, so (hopefully) does their work and responsibilites.
Artie is a marvelous Muppeteer. But he's not ready for the type of performance a Big Gun requires. Not yet. Someday maybe (forget "maybe" - i have no doubt he'll be there over time) but at this time he's "not ready for prime time". He's not right for Kermit at the presen. Is he the best choice out there? (Yeah, the "correct" answer is "No - Steve is" but if for whatever reason Steve can't do it of course) I certainly hope not! Maybe Disney tried and tried like no one in the history of mankind has tried something before and they really found Artie to be the best alternative. But considering the number of people who have worked with the Muppets (or even puppeteers who haven't), surely there would be a larger number of people who have the more finely honed skills such a role requires.