The Blockchain (proper noun) is the "best" chain (of blocks containing digital signatures) that originates from the Genesis Block (proper noun).  A proper noun refers to a "unique entity."  A chain of blocks that does not originate from the Genesis Block cannot be the Blockchain by definition.  

Consensus about the Genesis Block is more a question for linguistics than computer science.  Just like gold is that shiny yellow metal with atomic number 79, the Genesis Block is that collection of bytes with the message "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks" encoded and with hash 00000000839a8e6886ab5951d76f411475428afc90947ee320161bbf18eb6048.  

Both gold and the Blockchain have certain properties, as shown below.  Arguments like "if we all agree to start calling different things bitcoin, the blockchain and the genesis block, then that's all that matters," is again linguistics:  people could start calling the alloy of copper and zinc (i.e, brass) "gold" instead.  If everyone does this, then its name might eventually become "gold."  But this will never happen because it is useful for words to have specific meanings, and even if it did happen it doesn't change the fact that "gold" (copper/zinc) still isn't gold (element 79).