I don't think it will be possible to predict the future value of a cryptocurrency either, but gold seems to be the proxy for the direction, if not magnitude. Gold isn't significantly controlled by governments, but due to hoarding, the intrinsic value of gold (by actual consumers of gold) has been outpaced by the trade value of it for decades, if not centuries. As long as inflationary currencies inflate, deflationary currencies will be around. And I would assume that a mathematical guarantee of fixed supply would be more valuable to those people than monetary policy by gold miners.

Not that I think bitcoin and others are truly deflationary, as the presence of derivatives and fractional reserve deposits means that overall they could still be inflationary, in the same way that M0 could experience deflation while M2 experiences inflation.