Any cryptocurrency that succeeds in replacing Bitcoin will use the Bitcoin blockchain. Here's why:
Suppose a new cryptocurrency comes out which is a significant advancement over Bitcoin (something no existing altcoin can claim), call it Bitcoin2. Let's say Bitcoin2 uses a new blockchain. People invested in Bitcoin realize Bitcoin2 is superior and realize they should do something. They fork Bitcoin2 into a new cryptocurrency called Bitcoin3 which is identical to Bitcoin2, except it preserves the original Bitcoin blockchain.
Now people have a choice as to whether to use Bitcoin2 or Bitcoin3. Which one will people use? The people who are working on building the current Bitcoin infrastructure are heavily invested in the existing blockchain. These are some of the smartest cryptocurrency experts/developers/entrepreneurs in the world, and the cryptocurrency that they back will have a huge advantage. Businesses and consumers will feel more comfortable with a currency developed and supported by the same people and businesses who brought us Bitcoin if there's no longer any technical reason to favor another coin. So Bitcoin3 will win out.
Another way to think of this is: is it harder for the smart people working on Bitcoin to clone a technical advancement from another coin, or is it harder for the founders of another coin to build something replicating Bitcoin's infrastructure and community?
