We used to believe it's true, but it has come under question. In fact just recently there was a paper that concluded language is more a tool for communication than thought.

There are language that have lived for centuries without any lexical rules. Any value lost is only in the the eyes of the preceptivists. Language can and will easily live on with all the internal inconsistencies. In fact language rules are usually built around those inconsistencies because the language people speak is more important than whatever rules purists come up with.

I don't think designing a writing system is prescriptivist -- or at least if it's any good then it isn't.

Here's a good and recent example, a new, unified, orthography for Low Saxon. The way they did it is to take Old Saxon, re-trace the sound changes in modern dialects phoneme by phoneme and then assign glyphs to everything, which may be realised differently depending on dialect, say "sk" can be pronounced (English orthography) sk, shk, or sh. Mergers etc. are preserved in the over-regional orthography, though there's also a set of regular changes you can make to that universal orthography to get at dialect orthographies.

That is, what it's doing is simply to try its best to do both the history and the present of the language justice, to preserve nuance while providing regularity. Situations such as "loose" vs "lose" are perfectly fine because you derive the spelling of the adjectives from their roots (at least I assume why it's that way in English). Your dialect may, or may not, merge them in that situation the orthography doesn't say that you should or shouldn't do that -- only that you should still distinguish it in writing because there's people who don't merge stuff like that. People don't start to write "cot" and "caught" the same all of a sudden, either, they probably don't even realise they're pronouncing them the same.

Contrast that with the previous "standard" orthography (for the dialects within Germany, that is), Sass. It basically says "write it like you would write German" and that's right-out disastrous as German orthography just can't deal with the phonetics, merging and shifting especially vowels left and right away from what anyone is speaking. As such it was prescriptive by negligence, confusing many a learner.

People are taught how to write in an academic style in school.

There's different applications for writing. Some examples in include but are not limited to academic, creative, and conversational/casual writing. Education tends to focus on academic(correct grammar, writing essays, doing research papers) followed by creative writing which is storytelling and poetry.

Expecting people to write academically in casual settings is weird. If I write a letter or email to my friend it's going to include slang and have looser grammar.

Casual and academic writing have their place and time. Suggesting casual writing is inferior is just silly. If people always wrote everything like some formal essay it'd be fucking boring. No humor, no colorful language. Just boring ass academic writing everywhere.

Plus, believe it or not some of my most brilliant professors had a very casual way of speaking and writing. Academic use of language does not indicate intelligence. It just means they know how to write in an academic setting. Nothing more, nothing less.

The English they think is perfectly correct proper English would make the language prescriptivist from a couple centuries ago puke and kick them in the groin