The supply isn't really known. As of this older 2020 article [0], something like half of the total number of (future) Bitcoins haven't moved in at least a year. I've seen figures like 2/3 from later in 2020. Some fraction of those probably represent lost keys, and hence will never move again absent a consensus change. Another fraction represent people who are just sitting on the coins waiting for a chance to sell. But we don't know what the fractions are! These are big uncertainties.
10%, 50%, 80% doesn't matter. Any node who wants to change Bitcoin's code and rules is free to do so.
It just creates an additional coin and it already has happened dozen of times (Bitcoin Gold, Bitcoin SV, Bitcoin ABC, Bitcoin Cash, ...). Some of them claim to be "the real Bitcoin" but market consensus seems to disagree: All of them are either dead or valued at <1/100 of Bitcoin.
As long as you run your own node you define for yourself what Bitcoin is.
You are wrong. > 50% of mining can only censor transaction.
Changing the fixed cap would break consensus, so it can only be done with an overwhelming majority not only of miners, but also users, exchanges and merchants. (otherwise miners would mine an empty chain).
But doesn't the value of BTC go up as the reward halves? So "3 more halving cycles" should be counteracted by each coin being 8 times more valuable, no? I could very well be wrong, but that's what seems intuitive to me (I don't follow crypto very closely).
[0] https://www.coindesk.com/10m-bitcoins-havent-moved-in-more-t...