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.
That's true
provided this is a manageable part of an investment portfolio. It's not entirely unlike drawing to an outside strait or four cards to a flush in poker. You have a ~37% chance of tripling your money or more in a three way pot. I take that bet every time IF I have a bankroll big enough to handle the expected ~63% of hands it loses.
A more extreme example is a lottery ticket. If a lottery ticket costs two dollars, with a chance of winning of One in a Million with a payout of of $1M, then the expected return of any given lottery ticket is 50 cents. A terrible bet. But if that same lottery ticket with the same odds and the same payout cost 25 cents, it's a good deal, even if the chance of winning is extremely small. Just don't put a large percentage of your portfolio in lottery tickets.
I just had a chuckle imagining someone trying to martingale the lotto Lol. "I'm on a 16 week losing streak and i'm sitting on 65000 lotto tickets! I'm bound to win next week!"