Why do we need to capitalize “I” and the days of the weeks in English?

No disagreement with the answers here. I’ll philosophise a bit more generally:

Each language authority or community ends up with a particular set of conventions about punctuation and capitalisation—or borrows them from a more prestigious language. You only become aware of alternate ways of doing things if you’re exposed to other communities. And it only becomes a problem if your language does not align to a single community. (This is a problem for languages like Esperanto or Ancient Greek, which is often capitalised and punctuated differently according to the country printing the text.)

The conventions about punctuation and capitalisation are memes, and they are only partly selected in response to functional needs.

So you can argue that I was capitalised because i looks odd (and ambiguous with Roman number 1); note that the older and dialectal variant ich (which shows up in King Lear) is not capitalised. And capitalising nouns does make some sense as a disambiguation, especially in a language with zero morphology like English.

But mostly it’s just a matter of fashion. It’s also an in-group marker: this person follows the community’s rules, so they are declaring allegiance to our community.

Fashion does change (English got rid of capitalised nouns); but conscious innovation by a single person does not usually get far.

As someone whose other language (Greek) borrowed its punctuation from French, I cannot tell you how much better English would be if it used French’s quotation dash. All those stupid “he said”s and “she said”s and “he expostulated” would just disappear: the dash would tell you all you need to know. Joyce thought the same thing, and he used quotation dashes.

And no one else in English does, because the quotation mark and “he said” convention has stuck.

And why has the convention stuck? Why don’t you get to be a mountain goat instead of a sheep?

Because language use does not usually reward the unconventional. People just scratch their heads and think you’re odd. Just like no one got fired for buying IBM, no one gets fired for following the language community’s rules.

You can violate the community’s rules to make a point. If you’re a poet, or an entertaining storyteller, you may be heard appreciatively. If the medium is part of your message—if you’re foregrounding the language, you can start your own memes, and be an agent in language change.

But much of the time, and particularly in formal writing, no one cares about how witty or innovative you are. They just want the content, and then they move on. So being a mountain goat in most contexts just makes you annoying. And wit and idiosyncrasy are a currency you should use sparingly in any case: the more persistently you do it, the less effective your deviations become.

Why do we capitalize words after a punctuation mark? Is the redundancy necessary?

I agree with Susan Sisson’s answer, and refer you to Nick Nicholas’ answer to Why do we need to capitalize “I” and the days of the weeks in English?

With reference to OP’s rather defensive comment:

You can do as you please. And in turn, we get to react as we please. It doesn’t hurt our brain when you deviate from the community norm. We just get to think you’re an oddball. Which, despite the UK’s celebration of eccentrics, does mean your message ends up less, not more effective. And again: there are areas of life where innovation and rule-breaking advance humanity. Punctuation is not one of them.

What is the meaning of words – pluviophile , bibliophile?Why are they not mentioned in my latest Oxford dictionary?

Category:English words suffixed with -phile from Wiktionary.

Both your terms happen to be defined there:

pluviophile – Wiktionary : (biology) Any organism that thrives in conditions of heavy rainfall; One who loves rain, a rain-lover (from Latin pluvia)

bibliophile – Wiktionary : One who loves books; One who collects books, not necessarily due to any interest in reading them. (from Greek biblion)

If you know a bit of Greek or Latin (or Italian), you can work these out.

Why are they not in your Oxford dictionary? Depends on your Oxford dictionary. There’s lots of vocabulary that is considered obscure, specialised, or antiquated, and the smaller the dictionary, the more of that kind of vocabulary it gets to exclude. I’d be surprised if pluviophile gets used seriously now outside of biology, and within biology it may be antiquated. I’d have thought bibliophile is slightly less obscure, but I’m not writing marketable dictionaries.

Is it true that some non-American children who watch American TV shows have adopted that accent?

My parents were first generation immigrants to Australia. My mother had no English when she came here. My father had high school English, but no Australian accent. My parents worked in their fish and chip shop attached to the house, so much of the daytime I was reared by Sesame Street; I interacted with my parents in the evening.

Outcome 1. I did not speak until I was two years old. I presume that was because I was getting mixed English and Greek from them. I was reading pretty early, but not speaking.

Outcome 2. My parents asked their GP what to do about me not speaking. This being the ’70s, the advice was to use just one language (not: one language per context or per parent). My parents (to my surprise now) chose English. Had we not moved back to Greece when I was 8, I would never really have learned Greek.

Outcome 3. I did not really interact with any native  speakers of English until I went to kindergarten. Therefore, having been reared by Sesame Street, unsurprisingly I spoke with an American accent. In fact I did not acquire an Australian accent until after we moved back to Australia, when I was 12. My first year back of schooling I was teased a fair bit about the accent.

(Long gone now, though after three years of living in the States at 28, I was starting to do American things with my vocal equipment, and they come back if I speak for ten minutes with an American.)

Did I read in an answer somewhere here on Quora that spoken and written literary French are in fact two different languages — so much they differ?

Yes you have:

Andrew McKenzie’s answer to Why are written and spoken French so different?

To be fair, Andrew wasn’t making the claim explicitly, he says that some linguists would. He then lays out the arguments why you would.  And having a different underlying syntax is not a register difference, similar to English or other languages I know, contra what Riley Ertel says. It is a huge structural difference. Different language family-level structural difference.