Most people would agree that live shows need to be periodically updated to remain "fresh". The quality of the replacement is obviously yet to be determined (it will probably be shorter so that may be "worse" depending on your point of view) but changing a theater performance is something Disney should be doing with some frequency.
They need to turn their attention to DHS for such things as well.
I would argue that Finding Nemo (and Festival of the Lion King) are both timeless due to the thought, style, and budget they put into those shows. Voyage of the Little Mermaid, Beauty and the Beast, and Indiana Jones have not aged well. The Hunchback show would be an example of a timeless show. Or Aladdin: A Musical Spectacular. Broadway shows last for years and years and years in New York. The same can be done in a theme park if the show is produced correctly.
Broadway-style musicals are always the best options. The reason Beauty & the Beast feels dated is because it was originally supposed to be temporary, and it feels just like that. A long overdue temporary show.
Voyage and Indy are dated because they have dated technical aspect. For Indy, the old film cameras and whatnot drag/age the show, and it no longer really makes sense with the park's theme anymore. Voyage uses a bunch of old tech that doesn't really work anymore like the old film projector and the lasers (although I did like the lasers).
Nemo, Frozen, Aladdin (still performed on cruise lines), and Lion King are the theme park equivalent to long-running broadway shows. And that's something Disney has had an arm up on Universal (When Universal's longest running shows are Beetlejuice and Sinbad, both of which are permanently shuttered now... The Horror Make-Up Show is a prime example of a show that can stick around for decades and not get old or tired.)
ONE more thing... Just because you've seen a show a bunch of times, doesn't mean that everyone else has, too. Disney World mostly gets first-time guests, from what I understand, in comparison to Disneyland.