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). Because I think it is almost inevitable, just like all the other new technologies that just were better than the previous way of doing things.
you are right, one aspect nevertheless: there could be a successor to bitcoin in its current form ... then it's clear that everyone will jump ship. i think, quite a few of those who have "missed" bitcoin so far think, that they'll simply wait for this new incarnation and hold back their investment.