While crypto's obviously have a very high volatility, I think it's useful to compare their market cap to the fiat money supply. US "M2" money supply (roughly speaking, money in circulation as well as in liquid bank accounts) is currently $13,600B, or roughly 85X more than all crypto's. That's just USD; the Euro M2 is ~$13,300B USD. So just those two currencies are 166x larger. There's a pretty decent amount of room to grow from that standpoint.