I think this has been covered, although perhaps not explicitly in relation to this question. It doesn't apply to all art because for the most part, art is assigned value by the observer. We see a painting we like and think it has value to us therefore. Now marketing may make us like it more or less because humans are like that. But we still have assigned it value ourselves. For Modern Art, very often (predominantly I would say), the value is entirely the result of marketing. As Angillion says: there are a million unmade beds in this country. Marketing is what made Emin's one suddenly saleable for a fortune. We can deplore the outsourcing of what we like to the appointed arbiters of taste and personally, I think that's worth deploring. But whether you do or not, it marks Modern Art as different. We're not talking about people who might look at a Monet and not like it - that's again a person setting their own tastes. We're talking about something where the liking or disliking is determined by the marketing wholly.
If art is assigned its value by the observer, and an observer deems it valuable, then who is anyone else to say that it has no value? The liking or disliking is determined by the observer, not the marketing wholly. They may be informed by the marketing, but ultimately they still have to make the decision as to whether they believe it's valuable. You deem the marketing to be BS, so you deem the artwork worthless but as long as there are enough people that decide it is valuable, then it will be worth something. Call it a con, call it 'the emperor's new clothes', but that's just an opinion.
It's exactly like looking at a Monet and deciding whether you like it or not. Art is subjective; whether it's Michaelangelo's David, or Turner's 'The Fighting Temeraire', or Piet Mondrian's 'Composition with Yellow, Blue and Red' or Damien Hirst's 'The Physical Impossibility of Death in the Mind of Someone Living' (the shark). In every case, the piece is valuable because enough people have deemed it valuable, even though some people like it and some don't.
I'm again going to emphasise my point from earlier that Abstract Art =/= Modern Art. I like a lot of abstract art. These are abstract but I like them: They'd look quite nice on my bookshelf.
A bicycle against a wall? Noooooot so much.
I know there's overlap between Abstract and Modern Art, but the distinction is worth maintaining or this discussion will get very messy.
I think
@cheesyboy's definitions are quite useful here in order to avoid confusion. My point earlier was that 'Modern Art' (let's now refer to it as 'Conceptual Art') is a progression and/or abstraction of 'Abstract Art', in terms of the linear development of Art History.
You may not want a bicycle against your wall, but there are people who do — again it's all subjective.
See, I don't see this clear lineage to Modern Art that you do. I see a fracture and a subversion of art. I think we've been able to give clear arguments on that which don't depend on our subjective "I like it / don't like it" personal tastes. And I think we all know we're saying Modern as in the school rather than contemporary. There is beautiful art produced every day. It just doesn't get the coverage Damien Hirst does.
The lineage comes from progressive schools of artists building on the work of those who came before them and trying to do new things and push boundaries, it's just an evolution of the medium rather than a subversion. I'm sure there were traditionalists who labelled Picasso 'fractious and subversive' in the early 20th Century.
Hirst and Emin went to Goldsmiths and The Royal College of Art respectively, where they will have been emersed in art history and culture, and combined that with their own experiences. This will also have introduced them to other people (like Saatchi) who helped them progress in their career, but this kind of thing isn't a phenomenon specific to the art world. There are no doubt tens or even hundreds of better artists for every Hirst or Emin that won't get the same recognition, but success in life often comes down to 'right place, right time' and 'who you know'.
You can disapprove of that (especially if you don't like the artwork) but I don't believe you've given a 'clear argument on that which doesn't depend on your subjective "I like it / don't like it" personal tastes' — it's all subjective and you clearly don't like it, but you can't seem to accept that other people do.