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.
We have a low probability of Bitcoin becoming an important store of value or used for international tranfers or online payments..
There is a low probability that it happens but the reward will indeed be huge if it does
A lot of traders make their trade then incite others to do the same trade by posting here