What will happen once Bitcoin is replaced by a clearly superior technology?
It is quite likely that cryptocurrencies and related tech will revolutionize many aspects of our lives. But that is also exactly why Bitcoin will decline. Its current value comes from speculation rather than utility. When actual business is done on the blockchain, the value created by that will surpass value from speculation so much that no consideration will be given to bitcoin and its technological baggage. Business will simply use a newer crypto because it is more economical. We'll use something that's faster, easier, cheaper, and less energy consuming.
When superior competitors exist, Bitcoin will simply have no inherent value. It will be like a government invalidating a currency, except it won't happen as fast.