1. Bitcoin is scarce
2. Bitcoin is decentralized
The biggest reason why Bitcoin will always remain valuable is its decentralized structure. Bitcoin laid the foundations for a new understanding and has been moving forward ever since. We've been lucky to witness this progress.
The biggest problem with today's economies is that they print money at will, and this free money comes back as inflation. This is not the case with Bitcoin. That's one of the things that makes Bitcoin valuable.