This piece more than any other gets used as a punchline about 'modern' music (by which the speaker usually means 'art music written in the middle two quarters of the 20th Century') being
really bizzare. And it is, but I think that's primarily because many composers treated their works more as a philosophical exercise attempting to address the question 'what even
is music' instead of entertainment. In that framework, John Cage was legitimately one of the most successful composers of the period - his works consistently push the boundaries of 'sound organized in time' in new and different ways. To my mind, 4'33" can be viewed as either taking the importance of the rests in music to the extreme, or recognizing that a live performance always exists in a space and that space becomes part of the performance (i.e., by only having the 'background sounds' of the performance to focus on, the audience becomes aware of their own part in the performance).
There is in fact more to the score of the work than just 'sit for 4 minutes and 33 seconds':
The problem with a lot of Cage's work is that, by pushing the boundaries
so far, he ends up writing a lot of works that can only be written once. There are a lot of symphonies out there - there are a lot of different ways to write symphonies. There's really only one way to write something like 4'33", and once he did it,
that's it - anyone who attempted to do the same thing again was going to be unable to differentiate their work from his. So while I believe Cage was quite successful, I believe he was also a 'dead end' in terms of developing new movements within the musical world.
Having said that, music
is ultimately entertainment, and it's rather hard to go out of the performance venue humming or whistling 4'33" or any number of other modern works - they have a
severe lack of catchy melodies.