I was with you until that very last line.
There is much to be said in favor of a well-constructed piece of writing. I don't know all the technical jargon and I'm sure I have some habitual mistakes or idiosyncrasies, but when I write, especially if it is to be a long piece, I want to construct it with care. I am learning, though, that that is very much a personal preference.
If someone makes all categories of mistakes you mention--spelling, usage, capitalization, and punctuation--yet still manages to get his point across, where is the harm? It grates on me in an aesthetic sense, but neither is every man a painter and musician. Why should I expect a level of mastery in writing that is not a given for other areas of skill? Writing is a creative endeavor; it happens also to be saddled with the task of carrying meaning in ways that art and music are not. Many if not most people don't know the basic rules of melodic, harmonic, and rhythmic structure in Western music, and we don't hold that against them. Many if not most people don't know the basic rules of color theory and...the other rules of painting which I am too ignorant about to even name, and we don't hold that against them either.
If the meaning suffers from an abundance of mistakes, then by all means ask for clarification and get fed up. I do. If the meaning is clear, then the hard-to-accept fact is that the language may be evolving in a way that is ugly to you and me. Someone else mentioned Chaucer, whose Canterbury Tales are a struggle to read in the original Middle English, and with spelling particularly offensive to my eyes. Is this contemporary shift meaningfully different? The rules for language are, or at least began as, descriptive rules. They tell us what the language was doing at the time. If people are no longer following the now-prescribed rules, and are doing so in a fairly consistent manner, I think it points not to their stupidity but to a semi-conscious decision that they don't care about the classical English rules and aesthetics, and are going to instead use what is convenient for them to use. I don't like it either, but it's a leap to go from "this person does not write with care like I do" to "this person is an idiot." By that measure, any foreigner who is just learning English, or has only learned enough to get by, is also an idiot, and that should be an obviously false conclusion.
So write with care; some of us appreciate it. If you are lucky enough to create any lasting works, you may be remembered for your skill. I have been impressed by the writings of William James in many ways, and his clarity of thought and writing is a big influence on how I write. Yet, now that I think about it, he used a mix of the formal and the casual that was in his own time regarded as unruly at best. That could prove to be instructive today.