It's a neat goal to keep an organ playing for hundreds of years. I just don't think that's related to the musical composition itself, which is not impressive to me. The fact that Cage added the phrase "as slow as possible" is not, in my opinion, musically interesting.

It would be analogous to writing a screenplay, adding the note "produce the film using as much money as possible," and then having someone attempt to do that. It's technically impressive to spend $500 million on a film production, sure, but that small note at the end of the screenplay is not cinematically interesting.

Cage created art that transcends music and you are rightfully noting that it is not that impressive when judged solely on musical merit.

It’s like saying a dodecahedron isn’t that impressive when viewed sitting on a 2D plane because it’s just a triangke and there are more interesting 2D shapes. True, but so reductive it’s tautological.