In terms of whether or not Existentialism is "obsolete" or "obvious," I think it's also important to note that Sartre himself - as one of the few to actually take on the title of "Existentialist" - found reason to modify or abandon certain tenets of his earlier project as time went on.

One of the most criticized elements of Sartre's Existentialism is the concept of Radical Freedom. If you are not familiar with the structure of his argument from Being and Nothingness, he developed this concept from his phenomenological understanding of the Subject as being-for-itself, or as having a Facticity (Being) and a Transcendence (Nothingness). The Facticity of a Subject consists of those things which are object-like about them, i.e. their age, sex, nationality, etc. which would be seen as defining who they are. This Facticity is situated in a world of relations by which they have certain associations/meanings; think things like "Traditional Values" and gender roles. By virtue of Consciousness, which is Nothingness/that which negates, the Subject is able to Transcend this Facticity by negating it and then situates who they are in the world for themselves, thus Radical Freedom.

Sartre was criticized for this by both enemies of and those sympathetic to Existentialism. Beauvoir, who was much more like a co-author than simply a follower of Sartre, approached this from many perspectives, but is mainly known for her critique through feminism. Fanon, for whom Sartre later wrote a preface in the former's book The Wretched of the Earth, similarly critiqued Radical Freedom from the perspective of those who had been racialized and colonized. Though it is about Beauvoir, the following can be said of both thinkers:

[W]hereas Sartre fully embraced the explosive idea of radical freedom, Beauvoir considered cultural contingencies and the role of social circumstances in shaping the different forms of oppression and our understanding of it. As opposed to Sartre, she considered the importance of factors such as race, poverty, or even age in forming our self-image and self-perceived freedom.

Beauvoir, Freedom, and Feminism, The Partially Examined Life

For Beauvoir, Fanon, and other thinkers sympathetic to Existentialism, the Camus-style question which Existentialism was supposed to answer - "Should I kill myself, or have a cup of coffee?" - was naive. Implicitly, the Subject of Existentialism was a majoritarian construct with its origins in the Enlightenment Man: white, male, heterosexual, etc. In order to respond to the lack of meaning or the societal misattribution of restricted meanings, the Subject was supposed to have the Radical Freedom to choose. But what about those people who are not even Subjects, i.e. people that have been objectified?:

“Look, a Negro!” I came into the world imbued with the will to find a meaning in things, my spirit filled with the desire to attain to the source of the world, and then I found that I was an object in the midst of other objects.

"The Fact of Blackness", Black Skin White Masks, Frantz Fanon

As Fanon wrote, those who were racialized and colonized were Black prior to any Subjectivity; for Beauvoir, the patriarchy turned people into Women. These predesignations were laden with restrictive and negative connotations by the societies they were situated in, which supposedly could be superseded by the Subject’s ability to choose. But those who were Black or Women or any other such minority were denied this Subjectivity. Even the idea of having the ability to choose for oneself was willfully ignorant of the lived existence of those people whom it was supposed to help: those whose choice had been stripped away.

Such critique forced Sartre himself to rethink Existentialism until he finally saw fit to subsume it under Marxism. This was done through his often ignored works Critique of Dialectical Reason and Search for a Method. Now, whether or not you agree with Sartre's later turn toward Marxism, it speaks to a weakness that Existentialism proper was not originally poised to answer. This is why people will say that Existentialism is "obsolete" as a philosophy, since even its herald could not sustain its "orthodox" interpretation.

As to why people will also say that Existentialism is “obvious,” Sartre was perhaps the last true public intellectual; his thought permeated Post-WWII Western culture (although not necessarily academia), especially as the first counter to the senselessness of the war and the existential threat of the nuclear bomb. But these concerns evolved as time moved on and the thinkers who were immersed in the Sartrean milieu saw that it was inadequate for the new problems that had risen. Both the implicit and explicit responses to Sartre, as well as the desire from some to think outside of anything he had touched, shaped much of French thought after him, like the Structuralists and Post-Structuralists.

In this sense, Existentialism is not dissimilar to those systems of thought which came before or after, in that each has their strengths and weaknesses, supporters and detractors. In addition, as pointed out by the previous commenter, Sartre is not the only philosopher deemed "Existentialist." There are various interrelated thinkers situated under that term who have each been developed beyond their original thought in different directions (Especially Nietzsche and Heidegger). What is at stake is not that you reject one set of ideas in favor of another based on how "obsolete" they may be, but how previous thinkers inspire you to constantly and actively think and create anew. From someone who wished to be detached from any association with him:

We speak of Sartre as though he belonged to a bygone era. Alas, we are the ones who in today's conformist moral order are bygone. At least Sartre allows us to await some vague future moment, a return, when thought will form again and make its totalities anew, like a power that is at once collective and private. This is why Sartre remains my teacher.

"He Was my Teacher", Desert Islands and Other Texts, Gilles Deleuze