Well, no. Flatus is not a symphony - unless the term symphony (used unironically) has expanded a LOT greater than I'm aware.
I would even argue that 4'33" wouldn't be a symphony, in a sense of a 5-mvt work with differing tempi.
There are objective standards - a baroque chorale cannot be composed atonally - but a chorale of itself might be. This is where SSC's right - eventually, its the composer's intent matters. However, if that intent is lost on the audience, then the piece is less successful in that respect - it does not evoke, in a general sense, the intent causing it.
If evocation is not the intent of the composer, then it cannot be used as a criterion. On the other hand, other criteria must be used: was the song perceived to have a "buildup" at a certain point; did the feel perceptibly change; was what the composer was attempting successful in objective eyes?
There's this local New Orleans artist,
George Rodrigue who paints mostly three things: trees, cajun people, and a blue dog. He styles his people in a very specific way, all the time. This, to him, evokes a certain mindset of the cajun people. Ignoring the fact that I think his work is largely boring, repetitive, and bleak.I can see what he sees as bringing this out, therefore it is a successful piece.