[oleganza]

The more people buying up bitcoins, the bigger the price is, the more early buyers sell some portion of their stash, the more bitcoins dissolve into the market. If at some point, everybody who wanted to invest, already done so, the price will stop rising. Some guys will sell, some others will buy more and realize that now much more people own some bitcoins and are ready to participate in exchange.

Therefore, "investors" build a market among themselves as a side effect of "hoarding". And since there will be non-zero amount of people willing to exchange directly rather than sell, the price will rise further, attracting even more people. And so on until everybody who needs BTC gets some. And then nobody would need fiat.