This is so complicated. What follows represents the views as well as the written language of someone of a particular age and education (minus typos, of course).
Writing standardizes language as do the current mass media. The standards of written language have always been a bit out of sync with what people are actually doing when they speak because it used to take a while for new usage to be shared widely enough to make it into print. An example is “graduated high school” for “graduated *from* high school.” The former sounds odd to older English speakers because we don’t use “graduate” as a transitive verb. Another is “couple friends” rather than a “couple *of* friends (Can we say “pair pants”? we argue). Because American English reduces, sometimes eliminating, the stress on the prepositions “from” and “of,” what you hear is not what you write (“couple of” becomes “coupl’a” becomes “couple”). Both of these are now seen in print regularly and are becoming common usage. What causes people to point fingers at the internet is that the process of sharing and translating spoken forms into written language has been hyper-accelerated.
English teachers normally spend a lot of time and effort teaching kids the difference between the language most of us write and the language they speak, and depending on the distance between the two, this can be very difficult and turn kids off writing altogether. This especially true when kids get the message that the language they speak is somehow “defective.” It is one way that we have maintained a standard language among different groups of speakers, or “speech communities” and among speakers of different generations and periods of history. It’s also a way we have marginalized people whose spoken language is significantly different from the standard written one.
One problem that relates to the transmission of historical information is that the further we get from the usage of a particular age, the harder it is to read its literature. France has dealt with that problem by officially standardizing the written language. Certain things just aren’t accepted. In spite of this, however, French is changing, and as far as I know, the French Academy can’t police the internet as easily as it does published writing. Maybe kids should be reading more Shakespeare but also more literature in various forms of English vernacular, listening to the language and reading it aloud in order to appreciate the different vocabularies, structures, sounds, and rhythms.
It’s important to understand that there are many forms of what we call a “language,” and this isn’t something that we are normally taught. As a college composition instructor, I do see it as my responsibility to teach students the current conventions because they will have to read and write across a lot of contexts. Early exposure to claims of what’s “correct” or “incorrect” makes it difficult for them to see the bigger picture. When I tell them that “based off of” just wasn’t a thing twenty years ago, they look at me in disbelief. They also struggle with the idea of audience: that unless you’re writing only for yourself, you need to imagine the perspective of a reader who may not use language the way that you do.
(I don’t think we learn sentence and phrase boundaries from speech. Supposedly, we have an intuitive knowledge of what linguists call the grammar of our language, but what this comprises seems to get narrower and narrower. Our knowledge of the so-called standard written language comes from prescriptions based on a structured analysis of what happens in conventional writing. The same goes for punctuation, which is pretty arbitrary, and as you note, the norms for this differ across time and culture. It’s unfortunate that *anyone* ever gets the message that being able to manage all this has anything to do with basic intelligence — or worse, “being right.”)
We need a standard language in order to communicate with each other, and this may be even more critical as we isolate ourselves into smaller social, cultural, generational, and professional speech communities. Understanding that writing is a form of communication is critical. I suppose there’s a difference between creative experimentation for its own sake and simply not believing in your message enough to shape it according to a reader’s expectations. The former can be a way of communicating what can’t be communicated otherwise or pushing readers to think less conventionally. At some level it interacts with the conventional language. The latter can be lazy, but it can also show alienation from the conventions themselves and those who foster them. It can be a way of demanding that a writer be read on his or her own terms, but it usually takes a lot of authority to get away with that.
Language is political. It’s a challenge for writing teachers to show respect for the idioms of their students while teaching them the conventions of a written language that will give them a certain kind of traction but by which they may be less than engaged or even have felt excluded.
Hopefully, the changes in the “standard” written language will allow us to better communicate across the boundaries we continue to create.