I give the benefit of the doubt that Marchel Duchamp and others genuinely believed in the artistic merit of their works and their enthusiasm made it easier to persuade people to agree with their viewpoints. The brief history of these artforms suggests that in the absence of their champions they don't resonate with artists but are still talked about because they stoke conversation about the nature of art. Maybe that was their original intent. I don't think these endeavors were profitable enough to suggest dishonesty over passion but if someone tries to sell you a moose head mounted over a toilet for 500k because it's "art" you may want to question their motives.
Good art typically stands on it's own and connects with people without the need to explain why it's significant. Salvador Dali's work is visually interesting even if you have no clue what any of it means. If I see a bunch of objects scattered around a room it looks like clutter. If I see a bunch of paint splattered to a canvas if looks like a mess. If someone has to explain why these things are "art" is it the art that's compelling or their argument? The exception to that would be optical illusions.
Art is somewhat subjective so someone will always argue their own preference but if anything is art because "it's all subjective" then nothing is art. Buttering my toast in the morning could be art because I spread it using a spoon instead of a knife and that is an expression of rebellion against societal norms. Did I sell you on my art or my argument and is the latter what we want?