If you don't understand why the limit is fixed at 21 million, then maybe you should step away from Bitcoin because it is part of the foundation and contributes massively to the price. People who hold it are more comfortable because the supply is restricted and rather low. It also helps that it is a relatively low number, because it helps to beat inflation and there is a natural attrition rate of lost coins which will help. Why does anything have value in life? Because people have agreed to trade a good or service, in return for another good or service - it is how currency works. Instead of finding someone who wants your good or service, you convert it into a unit which can be traded in a general market.
Some things have value because people trust in them as a medium of exchange or store of value. Even though Bitcoin is often described as "Magic Internet Money", its value is based on the rules of supply and demand. That, and also the fact that Bitcoin is the most decentralized and censorship-resistant kind of money in the world, makes it an extremely valuable cryptocurrency. If the OP is asking why Bitcoin can't be based on something that has value, then he/she doesn't understand Bitcoin is all about. It's something commonly expressed by people who either hate Bitcoin or lack knowledge of it.
Bitcoin's been doing well on its own (despite the ups and downs in Fiat terms), so I'd say it doesn't need to be backed by anything to hold its value for a very long time. 21 million coins will ever be mined, making the pioneer cryptocurrency a truly scarce digital asset. If you believe in Bitcoin, then I'd suggest you "hodl" it no matter what. Otherwise, Bitcoin is not for you. With how far Bitcoin has gone since day one, it's unlikely it'll disappear (or go to zero) anytime soon. Just my thoughts
