I don't know that I'd agree that Existentialism is on a decline currently
, though it certainly has been
since the post 1940's era.
It seems to me that in the post-war years, at least in Western society, there was a strong social push toward conformity by the majority of people in the search for identity. Despite the boom of expressionism in music, the arts, and design, the prevailing norm of the majority was a cookie-cutter approach to individual values.
However, today, with the proliferation of the Internet, particularly the multi-media aspect of it, not only is information being rapidly deseminated and widely available, but on most levels of society, there is a desire toward personal expression that contains an existential element. This flow of unlimited information exposes people to a vast set of values, beliefs, and personal expressions that is ever-changing and omnipresent.
Exposure to this spectrum of expression lends itself to existential introspection on an individual level, and, as a result, we are seeing a rising proliferation of new philosophies, new takes on old philosophies, new splinter aspects of religions, and the merging of many previously-separate psychosocial and ontological systems.
Is this not the essence of existentialism?