Very good question, but I would like to add here is that the 21M will never be filled in my opinion. Why? because of BTC halving every 4 years, meaning that the amount that could be mined is halved every 4 years. The more halving cycles (4 years) we go through, the harder it will be to mine or "find" these coins, hence why I believe that the 21M will never be filled.
That was what I tried to cover in my question - I also think they will never all be mined because it will inevitably get to the point that it isn't worth anyone's time or money (electricity/hardware) mining for so long for such small reward - but as soon as the mining stops/slows significantly, so does the "blood flow" of the currency, as I understand it. Then everyone will be sat with bitcoin on their hard drives but no way of transferring them or doing anything with them


