Can't help but notice this thread popped up the same weekend as The Great Muppet Meltdown.
Obviously the online fan community uproar over the closing of a dated "screenz" attraction and the worst QSR pizza in Orlando was about more than the Muppets. I posit that it's Millennials realizing the great pop culture freeze they grew up in -- which started in the mid-90s, lasted until probably 2020ish -- is finally fading. People in their 30s and 40s realizing that top 40 radio won't always be Taylor, JT and I-V-VI-IV songs, that the biggest movie of the year might not always feature Marvel superheroes, fashion might finally evolve from Starbucks barista chic, and that the theme park rides they can spiel by heart won't always be there. They've been in-tune with what's "cool" they're whole lives, and suddenly the kids are shouting "Chicken Jockey!" and for the first time they don't get it and it's so dumb and for the first time they feel their age. "I was hip and with it ... but they changed what 'it' is!"
So they cling to the IPs they spent their whole lives with, most of which they inherited from Gen X anyway. The Muppets. BTTF. Ghostbusters. Jaws. But these are only nostalgia plays right now. In 10 to 15 years, they'll be even more dated -- and its likely that era's teens and 20-somethings will be clamoring for something not even on our radar at the moment.
This is the danger of single-IP-themed lands. "The American Frontier" is generic enough to evolve into what each generation needs it to be; Far Far Away will be forever tied to a 90s movie very much of its day. The Magic Kingdom of 1991 looked very different from that of 1971. Universal Studios of 2025 has exactly 1 ride in common with 2003 Theme parks need the freedom to grow and change. And becoming a nostalgia museum impedes that.
But... Muppets aren't going away? That specific show is, but Disney has plans to add new and substantial Muppets, Indiana Jones, Marvel, classic Disney Villain themed attractions across the next five years. A lot of their recent lineup has been early to mid 2000s era Pixar films, niche Gen X properties like Tron, or huge generational touchstones like Star Wars and Marvel (which you point to as being on the way out. Maybe for the films, but Guardians in the parks seem to be doing just fine.)
I don't disagree entirely with your overall point, but it's tough to believe the cultural moment is actually moving on when it very clearly isn't.
Disney shows how you can do both. Moana, Encanto, Frozen, Avengers: Infinity War and Endgame, Zootopia, Mandalorian have all been some of the biggest movies of the past decade for Disney across both kids and Gen X nerds, and they've gotten sizable additions to the park to match. Adding a Muppets or Indy ride doesn't mean you can't add an Encanto or Moana attraction, and sometimes you can make a Mando addition that everyone can enjoy. Sometimes you make a ride based on Snow White, or Oz, or Universal Monsters that is far from "relevant" but works as an incredible solid addition anyways.
Universal can 100% do the same. To some extent, that's exactly what they've been doing. Nintendo has been around forever, but Universal bought in back in 2015 when Nintendo's brand was in the pit from the Wii U era. We've seen a complete turnaround and now Super Mario is a huge kids property again, and we've got a huge new land to capitalize on it. We've got a Gabby's Dollhouse meet and greet and Shrek's swamp coexisting. It's hardly one or the other.