This is the grammatical equivalent of taking the post-modernist questioning of truth to an absurd extreme.
Just because all knowledge is subjective doesn't mean there is no objective truth.
Take physics for example. All our experience of physics is subjective. But we all share essentially the same subjectivity so it might as well be objective.
Newtonian physics worked until we expanded our empirical data - and hence our subjectivity - to the sub-atomic and interstellar levels.
Then we needed a new approach (quantum mechanics, Heisenberg, Einstein etc). But Newtonian physics still "works" within the lmiits of everyday human experience or subjectivity.
This is in stark contrast to the "social sciences" where we all have a different subjective position and it is not easy (or even possible) to determine objective truth.
That doesn't mean there is no objective social reality.
Similarly just because grammatical rules are arbitrary doesn't mean they are without function or value.
Language is arbitrary. There is no necessary relationship between words and their meaning.
Those relationships are only established through usage.
You can decide to use a word to mean something else entirely, but no one else will understand you if you do.
Similarly you can ignore the "rules" of grammar if you want to, but you risk being misunderstood if you do so recklessly.
However if you generally observe the rules, deliberately breaking them can be used for literary effect.