I think it's more that people don't believe there will be that much upside in the future, that most of the gains are in BTC's history. Late compared to what they could have been. In percent, that's of course true, as zero to any price is infinite percent, but in purchasing power per bitcoin, bar a discovery that p=np or some similarly catastrophic event, most likely not. 

 There is virtually no limit to fiat currency debasement 

 There is a limit.  Once it is so worthless they stop bothering to print make more.

That time will come to all FIAT.