To rationally decide to not buy a lot of bitcoins, you have to explain why it is impossible for Bitcoin to replace fiat (as a store of value, at least).
No you don't, you just need to explain why it is very unlikely. Just because something is possible doesn't mean it is probable. And I can think of a whole raft of reasons why it is improbable
Sorry if this point seems patronizing or insulting, but you need to be aware, in case you are not, of the concept of expected value. A very large reward at very low probability may have much higher expected value than a modest reward at high probability. Because the reward for owning bitcoin is so vast, in the fiat displacement case (which I agree is inevitable, albeit perhaps not with bitcoin as we know it today) even at vanishingly small probability it is a "rational" choice. The concept of rationality is debatable. But given the standard concept of rationality, the outcome is not really debatable.
...and by making this rational choice, people increase the odds of the positive outcome actually happening (the more people in, the higher we go and the nearer we are to the positive outcome).
It is quite marvellous, I don't know the actual invisible mechanism, but the visible proof is bitcoin price and its mind-boggling historical development.
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Anyone who thinks that Bitcoin has anything higher than 0.1% chance of success, should rationally buy bitcoins. The trick is that buying increases the chance of success. With a few iterations you realize that the chance of success is actually something between 25% and 75%. Then you are very willing to buy bitcoins. Which increases the price, rewards previous investors, gets more people interested, and makes the chance go up. In 3-5 years we have everybody using bitcoins, and an ordinary person has 5m
BTC in the west and 1m
BTC in BRIC.
Why was the adoption of a new technology with network effects never before so interesting?