I don't think that what modernists produce is best described as
"self-expression". Most of it doesn't express anything at all, or if it
does, it is something trivial like, "This is what green looks like," or, "I'm
able to pull the wool over people's eyes," or "Anything I say is art, is
art."
This notion comes from the idea that the most "sincere", "pure", and "real"
expressions are those which arise spontaneously from the subconscious
without any intervening thought, skill, mechanism, technique, or labor. The
truth is quite the other way. Without the proper means (technique, thought,
labor, etc.), the intended ends cannot be reached. Eliminating the means
doesn't make the ends easier to achieve, it makes them harder or impossible
to reach.
If you want to see "pure expression" without any skill or planning then
don't look at modern art, listen to the screaming of an upset baby.
