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.
That's the old "early bird gets the worm, but the second rat gets the cheese" argument. There is merit to that argument, but as long as the successor incorporates the Bitcoin block chain, it will not replace bitcoin. It will become Bitcoin. Green backs did not replace dollars after the U.S. War of Northern Aggression. Greenbacks became dollars.
If you missed the land boom, I suppose you could always wait for the real estate market on the moon, but that wouldn't make terrestrial holdings worthless.