Bitcoin is already the digital gold, the only problem right now is that it is a slow. With the current technology we have, you can't build a digital gold that is decentralized and fast, you just can't. That's way we will have second layer protocols like the Lightning Network and Sidechains.

No other cryptocurrency can replace Bitcoin as the digital gold. Let's say there is a new cryptocurrency that is superior to Bitcoin called Xcoin, if Xcoin became so popular that it replaces Bitcoin as the digital gold, then there is no reason that Ycoin or Zcoin with better tech to replace Xcoin as the digital gold in the future. What I'm trying to say is that the fact that another coin replaces Bitcoin as the digital gold will break all the concept of digital gold (the one), that's why it's either Bitcoin or none.