A monoculture will eventually fail, that is reason enough to keep alternatives.
If if there were some innovation with new coins, and not just a copy / paste of the old coins
If someone "hacks" (as inaccurate and unrealistic as that is) then every single Crypto-currency is screwed as they use SHA-256 in the address generation. So you could find another public key which hashes to the same address and steal all the owned coins of every coin that exists.
There is no defense in diversity if all the codebase are 99% copied from one.