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I am afraid I did not express myself clear enough so forgive me to repeat myself here.
Compared with property and stock, bitcoin is more like gold. That means 1 BTC is always 1 BTC, it will never collapse, burnt, bankrupt, or diluted. So it just like gold, can be a very good way to keep value. In short, you buy BTC and keep them in paper wallet, and after couple of years, its value will not affected by all the inflations, no matter how much paper money has been printed.
I am talking about the value, not the price measured by fiat. The value of BTC only disappears when it is no long used to keep value, just like the silver, which is almost just a useful metal now. If BTC is replaced by a better crypto-currency, then it's value may goes to 0. Otherwise, it is always there. No one will deny using crypto-currency to keep value is a great idea, and once it is accepted by the mainstream it is not reversible.
As we know, one of the big value of bitcoin is its scarce. There's only 21m coins, and that's the key for it to keep its value. Therefore, we buy some when they are still affordable and then keep them to hedge all the fiat inflation risks. That's the investment I mentioned. In this sense, holding BTC is safer and easier than holding stocks, properties, or gold.
Most of what you said above, however, are related to price and trading. That is completely short term and manipulated by whales. Concentrating on the price is speculation, not investment, and is not related to what I am talking.
One more thing to add:
In stead of asking 'is the price of bitcoin sustainable', how about asking 'is the value of fiat sustainable'? I am afraid the latter has much more obvious answer.
Fair play, sorry for misinterpreting.
Let's try again then
Compared with property and stock, bitcoin is more like gold. That means 1 BTC is always 1 BTC, it will never collapse, burnt, bankrupt, or diluted. So it just like gold, can be a very good way to keep value. In short, you buy BTC and keep them in paper wallet, and after couple of years, its value will not affected by all the inflations, no matter how much paper money has been printed.
Gold only has value as long as you can convert it into a currency. A lump of gold on it's own, for the most part, is useless. You can't go into 99% of shops and pay for things in gold, you can't eat it, you can't drink it. So gold is only has value by your definition as long as people are willing to trade it for something that is useful to you (e.g. fiat or food or an xbox.)
The exact same can be said about Bitcoin. It's only useful or has any value if people trade it, otherwise it's just useless hoarding of junk. So you're right, it is similar to gold in that sense.
So no we can look at your original argument that Bitcoin is safer than gold and all others as an investment (which was the point I was refuting in my original post you quoted.)
Gold is a tried and tested investment option (or store of value or whatever you want to call it) in so much as we have a better understanding of what it is, how it works, why it works, who wants it, how much we have, and who wants how much - it's part of our society and culture.
Cryptocurrencies are not. There are numerous things that could collapse the price of cryptos to near 0: a break of ECDSA & SHA-256, revelation that Satoshi & bitcoin = FBI sting, Satoshi dumps 2million coins, the internet stops working, a MtGox, bitstamp and BTCChina get hacked at the sametime and initiate a huge sell (like 2011.) If any of these happened, your cryptocurrencies would not be useful, because people would be considerably less likely to accept them as payment due to technical inadequacy or social stigma.
Society as a hole does not accept cryptocurrencies as valuable by default: e.g. it's not hardcoded into our society like the value of shiny gold. Psychologically, gold will always hold value to a large amount of people because it's part of our history and culture (even more so in other cultures.)
For this reason, I believe it would take a far more radical event to collapse gold to $0 than it would cryptocurrencies. And for that reason, I believe Bitcoin is a higher risk investment than gold.
Sure you can say if we all accepted and understood Bitcoin, then it would be worth something, and that may happen so it's a good investment. But that is pure speculation, and that's not the kind of value we're talking about, right?