you said that every muppet except for the earlier ones and the ones from saturday night live were there. actually, this is not true.
there are a lot of full-body characters not there. sweetums wasn't there, but that was probably so his ending appearance would be a surprise. The Mutations (three identical full-body monsters) were not there. neither was sam the robot, a sesame street character from 1972-1976 (the puppet should have still been in good condition by that time). Thig (thogs buddy from The great santa Clause Switch) and splurge (the purple monster from hey, Cinderella) also weren't there. I also didn't see any of the bad guys from The Muppet Musicians of bremen. I am not sure whether this scene was filmed before or after the sesame street season (i think the movie was filmed in 1978), but bruno (the garbage man who carried oscars trash can. he first appeared on sesame street in 1978) was not in that scene.
and most obviously, mr. snuffleupagus was not there. of course, this might have been because at the time of the movie nobody except big bird ever saw him. Sure, he could have been behind everybody, so that nobody could have seen him, but then the muppets in the audience were watching, and could have seen mr. snuffleupagus, although with the scene so crownded they might not have noticed and/ or thought he was just one of many weird-looking muppets in the background (but by that logic big birds friends should have believed that mr. snuffleupagus was real).
Do you know/ remember if Caroll Spinney performed Big Bird in that scene, or if he wasn't availible for that scene?