Self-expression, if taken completely literally, is a limiting term. Great art transcends the expression of the self. And if a great work of art is, in fact, self-expression, that is only because it demands that the self become more than a single individual (or even a group) with specific identifiable traits. The definition of the word "self" must be reconsidered and extended.
May we transform our self-expression into something closer to human expression, into a voice daring to speak for our naturally mute universe! Universal matter is a story told everywhere with no expectation of an audience. Consciousness is the surprise hit. There is no glory in advertising any product except the wonders (and, yes, horrors) of creation!
To reiterate, why (one reason) the modern decline of the arts? A: too much emphasis on the prefix "self" in self-expression. And a much too limited conception of what the "self" actually is. The self has been corrupted by a culture of individualism gone too far. Paradoxically, this leads to a creeping loss of a sense of self. Who are we? The question is equally hard to answer both when we are completely in someone's power and when we are completely, utterly free.
To ask yourself the question: who am I? Where do I come from? How do I find meaning in this world? Begs the question: Who are we? Where do we come from? If you don't get to the second set of questions from the first, you haven't yet properly transcended the self in self-expression. The same limitation applies to those whose "we" is too constrained - those whose "we" is really only an embellishment of the self - this group of people exactly like me... who look like me, etc., etc. If this is the case of the "we," the real questions still remain unasked.
For self-expression to mean anything, it must speak a language understandable to others. What good is a language that contains only a single word? And for all its uniqueness, what good is that word at all? Beware of turning yourself into the only word in your own, solitary language. But also beware of speaking some of the common, thoughtlessly secular languages of today that have no words in their vocabularies for greater, wondrous truths.