As you have said, maybe those ~4 million BTC can be accessed by those individuals, that's why maybe PlanB didn't include it in his calculation. And as he said, it will be subject for adjustment in the future research. So the model is not yet final, it's a working model, but so far, as we take a lot it seems that it is working as design by PlanB.

               This is why most of the people say that bitcoin's big advantage is also its huge disadvantage which is its total lost, hoarded, and circulating supply. But still it works well as a store of value and great investment specially for the people who have been in this industry for a long time along with those that keep on accumulating and holding their bitcoins. Although there can be no certainty that some coins that are missing from the scene are lost forever, the fact still remains; Bitcoin's supply is still a problem.

The theory of 4 million lost BTC comes from an analysis by a company that analyzed all coin addresses and concluded that any BTC that did not move from a particular address in a period of 4 or 5 years could be considered lost. This is of course a very dubious methodology that has proven to be very unreliable over the years when we could see that someone has been doing transactions with BTC that has not moved since 2010.

Even if we accept that 4 million BTC has been lost, we still have 17 million left (OP math is not very good because 21-4 is still 17). It is true that some people have admitted to losing hundreds or thousands of BTC over the years, but all of this together can hardly be expressed in millions - except perhaps some speculation about how much BTC was actually mined by Satoshi himself, and that number is always around 1 million. If I were to add to that another 1 million that others lost in total, my optimistic estimate is that no more than 2 million BTC was lost.

Todays' computer power would not stand a good chance at cracking the cryptographic encryption. The process could start but it would be futile. In the future where technological progress is close enough to provide immortality potential would stand a chance but it will also take a very long time. In other words, it is almost impossible to crack it.



Yeah, I've heard many people who got involved at the beginning (2009/2010) didn't really know what they were doing and needless to say did not take outmost care backing up their files, now left with no record and forever lost their access. It happens, and there's nothing you can do now.

Maybe I've just been here too long, but I no longer get that element of fun people seem to have when speculating about this anymore.  Feels a bit of a fruitless endeavour. 



Yeah, even if someone found that, if it has been exposed to the elements for roughly 8 years, it's difficult to imagine anything could be recovered from it.

Undoubtedly, there's no accurate way to measure how many coins are in circulation. However, the 4 million lost coin prediction looks quite possible. Likewise, chances are that most early adopters have lost the majority of their BTC in some forgotten hard drive.

On top of that, I also believe that all of us had some fractions of BTC lost throughout the years.

Brute forcing keys will never a thing on BTC. If it ever does, the network will do a hard fork and change its algorithm to protect itself.

I doubt that quantum computers will possess a threat to btc. If it ever does, unless you own hundreds of bitcoins, it will be the last thing you'll worry about.

Pretty much as you said - just coins which haven't been moved in a long time. To say that all coins which haven't been moved in >5 years are lost is wildly inaccurate at best. We see coins which haven't moved in over 5 years being transacted all the time, and we've seen messages being signed from address holding coins which haven't moved in over a decade, proving that they are not lost either.

The only coins which are provably lost and can be subtracted from the maximum supply are those which miners failed to claim in the first place, the 50 coins from the genesis block which cannot be spent, and those sent to OP_RETURN outputs. I am aware of about 130 BTC lost in the first of those categories, and about 27 BTC currently sent to OP_RETURN outputs, meaning the total amount of provably burned bitcoin is only around 200 BTC.

You could relatively safely include all bitcoin sent to obvious burner addresses, but once you start including coins which are inactive or you think belong to Satoshi, then your estimates become increasingly inaccurate.

A lot of the research around what could be cracked by quantum computers comes with a great unknown. It is definitely possible to a powerful enough computer would be able to crack any type of encryption because the amount of calculations it can undertake in a short space of time is staggering. However I'm not sure by that point it will be much to worry about, because it would make all kinds of essential services and financial systems vulnerable so nowhere would be safe. Ultimately some sort of failed-attempt time delay and two factor authentication system would need to be added to a cryptocurrency wallet to avoid such attacks.