I don't say harmony is good! Basically all music relies on at least some tensions of disharmony, and of course you can make music that is completely unharmonical or antiharmonical, just like you can make arrhythmic music or music with intentionally distorted rhythms to make listeners feel weirdly disoriented if that's what they like.
But letting go of the principles of harmony in favor of "subjective absolutism" seems kind of strange and I would even say harmful. It's like asking about architecture "Well, why should a building be comfortable and durable? If I prefer a dangerous and awful structure, isn't that just another valid subjective preference?" Well, okay, but so then what? Abandon teaching structural integrity?
It's pretty interesting to discuss the differences between arts, crafts, trades, and sciences... but these discussions are also often kind of stupidly polarized between objectivist and subjectivist views.
I had a friend who staunchly refused to learn any music theory, or any patterns of musical tradition, because they were attached to a subjectivist position on music. Actually I think it's a deep position that has to do with the individual's place in civilization! This person's music was outsider music, much of it really cool, but in some way fundamentally alienated.
One of my favorite books ever is Alistair MacIntyre's After Virtue, which (with a somewhat conservative flavor) argues that contemporary subjectivism is because enlightenment rationalism just fails as a basis for ethics, and that this is the reason for "the interminable character of contemporary debates." It makes the case for virtue within traditions and societal roles as the antidote for subjectivism, basically.
So typography is a tradition of virtue in our society, kind of, right? It's like, wow, there's this whole guild of craftspeople stretching back hundreds of years, who have developed a body of practice and theory that forms a coherent framework for font shapes and text layout; that's pretty cool!
Of course there are deep questions like "How are the traditions, skills, and theories of typography modified by the encounter with digital media?"
You could even ask "Is typography (or whatever other creative trade) a gated community of elite tastemakers who just prescribe their own aesthetics as if they were natural laws?"
But at the end of the day you have to look at the alphabet itself. That's not subjective. It's not really "objective" either. It's a cultural heritage and a common standard. If you mess around with it arbitrarily, people will think you're a weirdo, and they might not understand you at all.