I've never been on the Muppets so I can't really say if I'd miss it. Now you're all probably going "You've NEVER been on Muppets 3-D!?!?"
Now, back to Armageddon and Millionaire:
I saw some park attendance estimates on some MGM site and it showed MGM's attendence steadily decreasing sicne 2000. I know these are just estimates and not accurate, but I think this should at least tell you something.
Personally, I think they should remove soem of these older rides. Not nessecarily Millionaire, but things like Sounds Dangerous and Indy. I would like to see Sounds Dangerous replaced with Armageddon and Indy replaced with Disneyland's Indiana Jones ride. With those, they could leave Millionaire up for a few more years, then get an original attraction to replace it. I'd love to see sitcoms or game shows filmed in the Studios.