Preliminaries: ‘art’ is a word, attached to a concept (or number of concepts). If a word is used in a certain way, the concept attached to that word will reflect that usage. If you start to change how a word is used, the associated concept will start to change as well. Of course, you can reject a certain usage.
(One trick in science (i.e., figuring out the cause-and-effect processes in the universe) that has evolved is creating new words. Scientists typically do this by combining Latin or Greek base words to form a new compound. The advantage to this is that you don’t get confusing cases where existing concepts might be mistaken for the new concepts, the latter of which are created in light of new discoveries and theories about how that part of the universe works. (It’s not this simple.))
So, if there is a debate over whether something is art, the debate is not about what a word could mean (it could mean anything) or what it does mean (although this is relevant), but rather what it should and can mean. To ask what it should mean is to refer to some purpose for the word. To refer to a purpose is to ask what’s important about the world for us, and how a word might be used relevant to that.
So, a debate about a word like ‘art’ is often a debate about what is important about the world. What we think is important in the world is informed by what we believe to be true or real. So, if people have differing views on what is real, it isn’t surprising if they think a word should be used in different ways. The other main reason is if people have differing interests related to how a word is used – for example, if using a word one way helps one person, but is neutral or hinders another person, then their views on how the word ought to be used may diverge.
What is the primary importance of art? Is it ‘refocusing ideas‘? Or is it perhaps conveying notions of the Good – of Truth, Beauty, and Virtue? Where one stands on this to an extent will depend on what one thinks is real, and where one’s interests lie. For example, much of contemporary art holds to the idea of only subjective truth, is intentionally ugly (while saying it isn’t), and intentionally tries to destroy the sorts of things that would tend classically to be considered as virtuous.
So, if you think that a primary purpose of art is to explore, express, or better understand the Good, then much of contemporary art is either not art or is poorly done art. However, if you eschew these notions as ‘fuddy duddy’ ideas, or what have you, and instead think that the primary purpose of art is contextual novelty, say, then these sorts of works may be considered not only veritable art but art of high quality.
In the end, then, many debates over definitions come down to ontology (theories of what there is) and interest politics (what is advantageous to whom). Since many art critics, in this case, may be wrong as a group in terms of their ontology, and have certain interests which diverge from many others, it may not always be the soundest idea to listen to their theories of what art is supposed to be.