AFAIK, the purpose of Bitcoin being limited or having a supply of only twenty-one million is for scarcity reasons, and this is the same as gold, but even so, I think there are still differences between both of them, namely that Bitcoin's supply is already fixed, whereas gold, we know that it is still unknown, and gold can still be found more. In addition, we can even assume the supply of Bitcoin itself is less than twenty-one million, because we know that there is a possibility that some Bitcoin owners actually lose it for various reasons, and one of the reasons is no longer having access to their wallets.

As a reference to your question, please see the following: Cointelegraph - Can Bitcoin’s hard cap of 21 million be changed?